Try the regular 3.5mm cable, rotate the cables around a bit and see if it's properly plugged in. Try them with your phone/other devices or directly (without using the FiiO K3/anything else) into your front/back panel 3.5mm ports/plugs for the computer.

Make sure your glasses/hair/piercings/etc. are also not in the way, they may be affecting the sound.

For comfort with headphones, if you have a big head, ear piercings, glasses, et cetera, basically anything that will interfere with the seal/fit of the earpads, here's more info about headphones and glasses and workarounds it: https://www.reddit.com/r/HeadphoneAdvice/comments/zkjjcu/are_there_any_headsetsphones_that_are/j00kjt2/

Essentially try to isolate if it's a headphone problem as HiFiMan is infamous for quality control issues.

See here for a bit more info on HiFiMan quality control problems: https://www.reddit.com/r/HeadphoneAdvice/comments/zicl2e/most_common_defects_for_the_he400se/izrcf3p/

Otherwise yes it's probably just the expectations. Try IEMs instead as they are often better for value.

If you already tried parametric EQ, still go on squig.link and see if other sound signatures of headphones are to your liking, it's not gonna be like the other headphones but it should give you an idea.

See here for more info about Qudelix 5K, parametric EQ, squig.link comparison graphs, AutoEQ, et cetera: https://www.reddit.com/r/headphones/comments/109meb5/eq/j40cy1n/

Look up oratory1990 (check the subreddit, /r/oratory1990, don't forget the preset parametric EQ list (mainly for headphones but there's some IEMs there): https://www.reddit.com/r/oratory1990/wiki/index/list_of_presets) and crinacle's articles on what amps/DACs/etc. are and what they do.

More info on headphones/IEMs and PC gaming: https://www.reddit.com/r/HeadphoneAdvice/comments/1151ezm/looking_for_1st_real_pair_of_headphones_mostly/j8zm0q8/ and https://www.reddit.com/r/headphones/comments/10zxpfb/why_are_you_using_openback_headphones_for_gaming/j85vv18/ and https://www.reddit.com/r/HeadphoneAdvice/comments/10ks4tr/looking_for_iem_for_gaming_and_music_listening/j5sjhyh/

Open back headphones noise reduction (scroll to the bottom half part of the comment), earwax clogging issue, parametric EQ, noise reduction for your PC/room, et cetera: https://www.reddit.com/r/HeadphoneAdvice/comments/115c99b/coming_from_iems_and_looking_for_headphones_150_eu/j92mpe9/

I used the HE400SE with the FiiO UTWS3/5 TWS adapters and they were just fine too. Again just make sure HiFiMan quality control didn't strike you and it should be just fine.

And yes don't forget the library or genres variability part, for some people they need a bit more bass or say less treble with some headphones when it comes to certain songs/albums from genres and so try different songs from your usual library and compare it to your friend's Sundara/other audio stuff.

The other main recommended headphone in that price range is the Sennheiser HD 560S.

These days Sennheiser is always running them sales on their refurbished stock of the HD 560S/etc. and these are sometimes the new revised version (different cable/etc.), btw, but they are significantly cheaper. I've heard people are also able to get another (replacement) unit if their package is like clearly used/dirty/etc. but it's normal for the dust/etc. to be there though. Check /r/buildapcsales for those refurbished/renewed/used/etc. sales, it's usually with Best Buy and Amazon too.

How long have you had them? The brain takes some time to get acclimated to a new sound, and it's unlikely that a headphone will sound great to you right after getting it. Listen to it exclusively for a week, then go back and compare it to other headphones.

There's also the bit about expectations. I was sort of the same way when I first started getting into audiophile headphones. It's like "that's it?" I thought the changes would be huge, but over time I learned that it's not going to add new instruments to a song or anything like that, it's just going to be a small, incremental change. It also depends on what you're coming from and what sound you're used to, like in my first point.

And finally, yeah, you might just not like them. The sound might not be for you. Ears have different shapes, brains are wired differently (there's some research done when doing MRI scans of subjects where people hear different things from the same signal, and those differences are correlated with right-side vs left-side activity in certain parts of the brain [Schneider et al., 2005]), so two people can listen to the same headphone on the same amp/dac, and their brains can perceive two different sounds from them. I admit that I find it odd that someone would describe an open-back Hifiman as muddy and veiled, but I haven't heard the HE400SE myself, and my brain is not your brain. If you've given them a week and they still don't do it for you, I'd suggest returning them and getting something else, or just sticking with what you have and enjoy.

Can I try them?

You should return them and look for something that better suits your tastes. Are you locked in to that price range? The Sivga Oriole might be a better fit for you.

Do you listen electronic music by any chance?

Maybe you just don't like their sound signature (it's darker than the Sundara) Always check if you connected the jack properly I had times when something little was blocking it.

You can check your audio options maybe it has enhanced sound on or the quality set to CD or lower. Also it depends what on your music service lower quality means more compression that could make sound muddy.

When I had them I didn't have problems like you also your ears need time to get used to new headphones. Maybe you are looking for something v shaped around the same price you can buy a AKG K361 or 371 which has more bass and top end.

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What you are describing sounds like you are running the headphone off a lackluster dac/amp. I do not have a K3 to comment but I think I know what you are talking about.

I plugged my 400se into my PC and my EPOS GSX300 and achieved similar results. Volume for days but it just sounds like a shit headphone and there isn't a thing you can do to make it sound better. The headphone sounds congested, the bass is one note and everything else fairly mediocre. I would say that this is not a great headphone and I would have returned it for something else post haste.

Plug it into something a bit more robust. Bass becomes better textured, excellent control and extension but the punch/slam remains about the same strength which is on the gentler side as far as planars go. Bass excitement was never a quality of this headphone, so coming from gaming headphones this would be a let down and don't expect throwing more power at the headphone to help, it's just enough to keep the headphone from being lean and boring, I would say it is a lively sound. Mids become uncongested, voices sound organic with decent timbre, decent spacing and imaging. The soundstage seems fine, it's not a big improvement but I feel everything is where it is supposed to be. Excellent clarity in the mid to uppermid region, no notable issues here but seems about right for the 400 series.

I feel like there is a point where you can hit rock bottom with this headphone, however if after a week of listening it doesn't grow on you, most definitely return it for something else as there shouldn't be any reason for you to upgrade your gear around the 400se, just get a better headphone instead.

I feel like the 400se is a great starting point to get an idea of what the hifiman sound signature is all about but this headphone does need a little bit of finesse to have it all come together and I feel like sometimes I can get it all together on less but some of the biggest complaints I've seen is about the HP needing power (not for volume) but to bring out the best performance out of this headphone and this isn't coming from people who are inexperienced with headphones but from reviewers themselves. I would agree but I've had fairly mediocre dac/amp combos deliver the goods but if what you are saying is true, it sounds like the K3 might be the problem. Just my 2 cents.

Interestingly, the sound tonality should be almost the same: https://crinacle.com/graphs/headphones/graphtool/?share=IEF_Neutral_Target,Sundara_(2020),HE400se

I do not think 400se should be bad choice. There are more measurements taken of this headset and IIRC they only tend to have just a defect with a bit of narrow-band resonance around 700 Hz. Otherwise, the performance looked fine. So, I am quite at loss to explain what the problem is.

You should’ve bought the 560s instead, amazing deal for that price, sounds clearer than headphones 2-3x its price, altho some people say it sound too flat and thin, but it can be EQed for marvellous results

I think people rave about them for a few reasons. First. They are entry level planar. A lower priced pair of headphones that introduces people to something new and different.

Second. They may have been better before. I have a pair of he400i and sure, prices come down, but maybe they lost something on the newer versions. This wouldn't be the only time. Look up the he-6. They remade that one and it's no where near the original. Doesn't stop them from charging a lot for it...then you see the Adorama sale and start wondering why you can get them for so much less.

Hifiman has some weird pricing strategies that have kinda turned me off the brand. Though I have a few pairs of them, they aren't my favorite headphones.

So...here's the tricky thing about "mid-fi" and Hi-Fi headphones -- at a certain point, it's not about quality, it's about sound signature and your preferences. If a pair doesn't sound good to you, then just find another pair. Even if that pair is cheaper. Price doesn't mean much of anything. So don't feel bad. Return them or sell them and try something else.

Definitely try to go to a meetup or CanJam or store that has some to demo. You'll feel a lot better about your purchase and you may even be willing and confident in spending more.

I've found this hobby to be a slippery slope though.

I had a planar magnetic headphone. It sounded god awful from one dac/amp I had. Even though I could get them loud enough, no problem. Them I tried on a much more powerful dac/amp and they sounded awesome! I remember there being a website where you could calculate how much juice you needed for specific headphones. In my experience it's not about how loud you can get them, there are other specs at play. Planars are power hungry, and in my experience need a specific powerful dac/amp to get a full bodied and clean sound.

The k3 is most likely underpowering them. I would suggest the k5 pro. I had similar experience with my he560 when I first got them, upgraded the amp/dac and they came alive.

Sounds like the need a more powerful amp/DAC. Not really worth getting something more expensive though if you have these as your only headphones. I recommend returning them and getting something more sensitive and easier to drive

Maybe the amp is making it sound muffled/muddy

The fiio k3 isn't a balanced amp why would you have a balanced cable, rather switch to fiio k7 then

Edit : Sorry for the inconveniences. It has a 2.5mm balanced out

Try with an Apple dongle, it's cheap.Perso I hate Fiio, so I'm heavily biased toward them. Maybe it's the culprit, but maybe not.Or at least try on the amp of your friend who has the Sundara.