I also don't find the HD800 unlistenable, especially not at lower volumes, hence the reason I've still kept mine. But as you and others have mentioned, they're definitely treble tilted, and do sound a little bit less full or slightly cold. Even outside of music, testing instead in gaming or movies, the HD800's simply do not have quite enough bass nor the tonal balance to accurately represent the drama of things like V12 natural aspirated engine notes, clicks, cracks, pops and bangs, or explosions, gun shots, assault rifle bursts, and things similar. These sorts of effects and sounds come off as ever so slightly too muted, crisp and lacking in theatrics or realism with the HD800. On the plus side, they have an incredible amount of transparency and separation, and an absurdly spacious and wide soundstage.
Regarding the Z1R's, on reflection I'm not even sure I'd describe them as especially dark, since they still retain a somewhat healthy amount of high-end sizzle and sparkle, just not to the extent of some of the German cans. Warm is perhaps a better descriptor. Overall they sort of sound like one of the popular high end German cans (eg HD800S or T1) with added bass emphasis and body, and ever so slightly tamed treble. Though still retaining much of that imaging prominence and focus on detailing and soundstage.
I'd describe headphones like the LCD-2 and LCD-3 as more darker sounding since they not only have the aforementioned added bass or mid emphasis, but also notably rolled off or more relaxed treble, lacking in as much sparkle or energy, along with a more congested or less wide soundstage. The benefit of this sort of a darker presentation is the ability to enjoy a more sumptuous, smooth and unfatiguing listen.
Speaking of dark, warm, cold etc, I think this is one of the reasons I personally enjoy the Z1R's so much. If you want a high end, fuller or bass emphasised headphone, you could choose something quite dark sounding like the LCD-2 or LCD-3, but then you lose out on soundstage size and imaging somewhat. Alternately you could pick something more v-shaped like the TH900, but then you risk a higher chance of things being zingy, sounding a touch metallic, thin or hot up top. The Z1R's for me strike a really good balance between the two, falling somewhere directly in-between.
As mentioned earlier, I can't hear any sort of notable 10k spike in my MDR-Z1R's, they are generally free from the kind of temperamental treble that v-shaped, bassy headphones can exhibit, and likewise, they're also not quite as treble rolled off or congested as the two aforementioned Audeze's. That sort of balance was what I was looking for in a high end headphone, for years tbh. And whilst I still think $2000 is too much for a pair of headphones, at least they're not a ridiculous $4000 like the LCD-4's.