Okay I'm sold on Emotiva (with Quickie) with the HD800 as well. It's better than Crack. There I said it. Koiloco will agree with me that it pairs well with the Emotiva. It's something with our batch. We both bought from Amazon during the special $890 sale and the serial numbers are 25XXX to 26XXX. It's not sibilant at all. Koiloco and I have listened to the same HD800 that belonged to a mutual friend of ours, and we both thought it had sibilance and treble problems, but when we received ours today, those problems were nonexistent. We did some measurements to confirm this. My set peaks at 5.5KHz and then tapers off. Worn for hours now, no fatigue at all. In fact, I get more fatigue from HE-500 and its bass impact. Koiloco said he tried our mutual friend's HD800 with the Emotiva and it sounded horrible, which is a stark contrast to our HD800s we received today paired with the Emotiva.
Seriously wondering if Sennheiser did a recent revision.
One drawback is it seems to have a higher noise floor than HE-500 when paired with Emotiva.
Sonido, these new HD800s we have don't seem to have that obvious treble spike like in our common friend's unit. However, IMO, the Emotiva/HD800 combo is still pretty far from good synergy in comparison to amps such as WA22 and even your crack. Maybe with your chain, use the crack to control the output on the Emotiva to even things out a bit. Emotiva is great for HE500 but I still don't think it will be a good fit for HP such as the HD800.
I do agree with you about the fatigue improvement. My HD800 still sounds "steely" and "lifeless" as I remember from 2 weeks with Jing's but seems to be more tamed across the FR.