https://www.stereophile.com/content/innerfidelity-headphone-measurements-explained "You can also think of step response as a measure of frequency response where the leading edge slew rate indicates the high-frequency limit, and the length of time it can keep the step at the new level an indication of its low-frequency limit. At every point between, you can think of the level of the top of the step response as related to the frequency response at the frequency whose quarter wavelength is equal to the elapsed time since the leading edge of the step/square wave."

I would agree with u/SupOrSalad above and say you're probably just not used to the sound then. If they were faulty you might hear buzzing or one side sounding obviously different from the other. HD600 QC is really good to the point both channels are usually very closely matched, so it's highly unlikely you've got a dud pair.

If you're listening on your phone, turning the volume to max and it still sounds too quiet, this is a sign you need an external amp or need to swap them for a different pair which is easier to drive such as the HD58X or HD560S. I listen to my HD6XX from a Pixel 4A which according to a review on a famous audio forum is completely incapable of powering them... well I never need to turn them up to max volume even listening to classical with high dynamic range.

I hope you learn to love these headphones as many others do, then you will be able to keep them as a reference pair for the rest of your life.

Wow OP I'm sorry you've been downvoted for such a reasonable position, which is spending what you can on transducers, the most important thing (I expect to be downvoted for backing you up on this).

By "muddy" do you mean the bass frequencies sound too loud such that they are masking higher frequencies, or do you just mean that higher treble frequencies aren't as loud as you're used to? Because the former would be an unusual experience to have of the HD600, which has neutral bass actually rolling off in sub-bass, whilst the latter would be expected and why people describe them as "smooth" coming from other headphones which might have elevated treble.

Could you share some measurements? I'm still between 3-5dB under neutral between 1-4kHz. Have you done anything to attenuate the treble, because if not this could explain the 'cleaner' sound you're hearing: they're very elevated between 5 and 10kHz and I've found this is exacerbated by larger pads. The only way I've found to address this is to add damping material in front of the driver; the best thing I've found is 2 squares of very dense foam in opposite corners.

I finally tried the block button, coming from reading posts like this for a lifetime. I just can't go back anymore.