@Jacob
O2 stack with HD800 is not bad at all, but I found it lacking in dynamic contrast. Everything else was fine.
I've heard the Crack twice. The first was with the HD650 on Bottlehead's tape rig at RMAF and I didn't like it at all. The second time was a few weeks ago using HD650, DT990 and HE-500. I preferred and enjoyed the DT990. The HD650 was a distant 2nd and the HE-500 is not a good match with the Crack. I find the Crack to be a middling amp.
I've compared the HD600 and HD650 many times over the years, but always came away preferring the HD600. About 5 years ago my main rig was a Little Dot MKVI balanced with DACMagic and HD600. I preferred it over setups much more expensive and have never truly settled on a full size headphone I would own until the HD800 after going through an IEM-only phase and brief flirt with stats (O2 MK1).