You must be using the wrong XLR plugs and receptacles then. High quality Neutrik XLRs are super durable and have a very satisfying click when latched and are not at all wobbly. I've definitely had some 1/4 inch jacks get finicky on me when you move the plug around

They can also be hooked up in parallel with out too much issues since the paralleled impedance will still be plenty high

If you ever get the chance to I'd highly recommend you try the HE6SE v2 on a beefy class A amp. I was running mine off a topping L30 v2 for a bit while I was putting together my Beta 22 amp and it's night and day when it comes to bass clarity and impact driven off of pure class A with 5W into 50 ohms

The HFM HE6SE v2 is a absolute joy, but only if you can power it correctly. I'll pull out my old dx3 pro plus tomorrow (if I have power since there's a snow storm in my area) and try my He6se v2 with them. I had a little bit of fun with EQ and my He6se v2 and man they take EQ super well. I was able to get chest thumping levels of bass but needed a ton of power to get them up there. My beta 22 amp does like 5w at 50 ohms and I was really pushing the volume to 2-3 o'clock with a -40 db preamp gain along with a +40 db bass shelf. Obviously I don't have those settings all the time I just wanted to see how much bass I can get it while still having super clean and detailed bass. Oh also be warned they are pretty heavy with the double sided magnets and all (I think almost 1 lb/450 ish grams)

Probably a boatload of R&D and engineering. I doubt a gaming gear company for OP's headphones did much to compensate for this shape tho

If you look at your ears the ear canal is not centered on the D shape. You usually want the drivers to be more centered to the ear canal for best tuning (unless they were engineered to be different but that's a whole nother can of worms). The D shape cups move the drivers too far back

I think the stock cable just puts the 2 negative pins to ground