HD 400 PRO

veiled uppers, the smushed soundstage, a lack of low end rumble If those are your concerns with HD 6xx, you'll find Sennheiser HD 490 Pro suits more to your preferences, depending on how much bass we're talking about. Bass is stronger than the HD 6XX but if you want really strong bass, sounds like you have to go for planars or even some closed ones. I can only talk about headphones that I own, though. HD 560s or HD 400 Pro (they're identical) also have the kind of sound profile similar to HD 490 Pro and you can find good deals for them. I haven't heard the new HD 505 and HD 550. Fiio FT1 got great bass but you may find them a bit dark, seeing how you evaluated the treble on HD 6xx.

While 6\*\* series are great headphones, I wouldn't recommend them for gaming. Their pads do wear out over time, which affects the sound. I assume since you'll be using them for long hours, you might not get to enjoy them to the same degree for too long. And replacement pads aren't always available, nor they're cheap. 3rd party replacements are never the same as the originals. You also have the issue of being unable to swap the cable for one with a boom mic. My suggestion would be to go for one of these: HD560s, HD505, HD550 or HD400 Pro and then pair them with HD500 BAM to get the mic functionality, if you need it. Do consider HD490 Pro if you don't need a mic because that's both the most comfortable and the best sounding headphone that I own. You can use them for hours without any issues. Personal tastes differ, of course. Some still prefer the 600 series.

You could also consider the HD560s. Good for studio work, very accurate for the price, pretty good bass presence. They are basically a cheaper version of the HD400 Pro -cheaper due to plastic build, cheaper enclosures, but the same drivers as the HD400 Pro. They're also quite comfortable, your ears will most certainly fit inside the pads. I just got a pair of 560s to go with my HD620s closed backs. I'm very happy with the sound. My normal monitors are Neumann KH120A speakers, which are extremely accurate (I also have a treated room).

I’ve been happy with my Sennheiser HD-400 Pro. Feels like a neutral and balanced sound profile. Comfortable for long sessions. I believe they are basically the same as the consumer grade 560S which may be a little cheaper but perhaps with less durable hinges, etc. Depending on your location, I’d recommend searching the used market to see what pops up. If your environment allows for it, open back is best both for sound quality and reducing fatigue.

I have Seenheiser HD400, so a very budget model. They feel very cheap, but they survived more than my gaming headset made mostly from metal. Idk how, but they are sturdy

Still love my Sundara’s. The HD400s were my first planar’s and really blew my mind, but the Sundara are in a different league and for 2 years were really the only headphones I really used. Enjoy!

Second this. Got my pair of Sennheiser open-back headphones a while ago now and I’ve never looked for another pair since. It takes a while to get used to it but it’s produces gorgeous sound quality.

I like open-back Sennheisers. Very comfortable.