So, owning and loving the HD6XX aside from other obvious likely acoustical improvements with moving to the top of the Senn stack with these, would the HD8XX likely improve on the one major knock the 6XX/650's seem to get? That being a bit lacking on the bass end of things? TL:DR being would the HD8XX have noticeably more/fuller bass than the HD6XX has?

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Sep 1, 2021
Some info: ”Fuller bass” = wetter bass with an extended decay, which is a trade-off that masks some detail. The larger driver in the HD 8XX can easily displace a lot of air for a headphone, but it is of the more “impactful, tight, dry” variety than the “warm, wet, muddy” type. You can EQ lower the mids and highs so that the bass stands out as a “higher” quantity, but it’s still going to be tight bass. The HD 6xx doesn’t lack bass quantity from a recording studio reference point of view, but any clean/tight headphone can always add wetness (slower decay and 2nd and 3rd order harmonic resonance) with a DSP program. My HiBy R6 Pro and FiiO X7 portable music players both came with DSPs that would do this; I know there are PC apps that can do this sort of thing too, I used to do it with the software suite that came with my Creative Labs products (internal sound cards and external DSP/DAC/amp products), that’s basically what their “bass boost” slider did in their products.
Mar 4, 2021
If you loved the HD6xx, it is doubtful that you will fully enjoy the 800. I say this as an owner of both. The 800 has a completely different sound and can be considered a treble monster. Though the 8xx should tame it a bit, I wouldn't recommend unless you're looking for something different. If you want an upgrade with a similar sound signature, I agree with the ZMF Aeolus recommendation. The Focal Elear also has a similar response that I enjoyed and can be found used for around 500 USD.
According to Rtings measurements, the HD6XX have more bass than Focal Elear. It has some more high-bass but personally I think most headphones have too much of this (makes for a muddy, boomy sound). The low bass and bass extension is what is important in my opinion and is where open headphones is lacking, mainly. I have the Philips Fidelio X2HR (non-HR is the same), it's my second unit (soon to be third) due to quality issues (I love the headphones and they are probably the best bang for the buck you will ever find as they are regularly on sale, as they are being replaced by the inferior X3s. Anyway, the X2s have more bass than 6XX and better bass extension. So if only bass is considered, they are definitely the better alternative to the 6XX than Elear. Having said that, I think it would be crazy buying those if you already have the 6XX. I've tried both back to back and they sound quite similar. X2 have a little more bass and is more livelier, however 6XX sounds a bit cleaner. They are also more comfortable and quality is a lot higher. I still love my X2s, though. For the price, they are a no-brainer. You just have to know about the weak spots of it (joints, assembly screws getting loose), I've been through 2 now and looked inside them. Pad replacement is also a challenge. But they can be repaired with 3D-printed parts and that will be available with time. They'll last as long as you don't drop them and keep an eye on screws (retighten when lose) and make sure there's nothing pushing or hitting the female jack connector or hanging them by the head support (poor elastic strap lifespan). The angle of the connector out of the headphones is stupid, it will hit the chair back if you turn your head.
Comparing the 6XX to the 800S, the difference is mainly bass extension as well as a slight decrease in low bass. The amount of increased bass from the 8XX should be enough to make them comparable or even more bassy than the 6XX granted you compensate with EQ. It still have the same "distribution", although a slightly improved extension. The problem is getting a DAC with a good and detailed enough EQ, though. Or a properly implemented software EQ. I think EQs are underrated/easily forgotten. It really can do a lot to a headphone/speaker. I usually increase low bass (10 - 40 Hz) and perhaps decrease mid bass and usually decrease high-bass. Then adjust mid and treble. I find that most audio gear would benefit from less boom, better balanced mids and more clarity without making treble harsh. The number one challenge with headphones for me is getting every part right. The closest for me is open headphones, all I need now is bass extension and perhaps the stereo separation and soundstage of higher-end headphones. Hifiman is noticably better in this area, but bass is still an issue.
Mar 3, 2021
No. If you want fuller/more bass than HD6XX, you're looking at the wrong headphone. I will point you to the ZMF Aeolus instead.