I recently responded to another Head-fi'er regarding the comparison between the Schiit Valhalla 2 and Bottlehead Crack w/Speedball upgrade. I thought I'd share that here.
The Bottlehead Crack and the Valhalla 2 are both arguably the best under $1K-1.5K amps for the HD800.
Do keep in mind, both of these amplifiers are OTL (output transformerless headphone amps).
Let's assume that the Bottlehead Crack was preassembled, as I think you already know it is a DIY amp that must be assembled by you. There is no question that the Speedball upgrade is necessary and crucial for pairing with the HD650 or HD800.
The Bottlehead Crack is as stereotypical as a tube amp gets: It gives a warm, smooth tone to the HD800. To my ears, the warmth was too much for the dark sound of HD650, but still a great pairing. The crack has an output impedance of 120 ohms, which is massive. This is an amp only meant to be used with 150ohm and above headphones, no questions. Compared to higher priced options, it is not as resolving and controlled enough to take full advantage of the HD800 capabilities, but almost there. The choice of tubes in this amp, given it is entire tube driven, plays an important role in how your sound is coloured.
The Valhalla 2 is a whole other story. This is not a typical fully tube driven amp. It is very resolving, even for HD800. It has fast, neutral sound, and the clarity is astonishing at this tier of amps. The Crack is definitely a few notches warmer. I prefer the HD800 with Valhalla 2, as it takes advantage of all the capabilities of the HD800. The output impedance is 3.5 ohm on low, and 14 ohm on high setting, so it pairs a lot better with other headphones, though I do not recommend this with low impedance orthodynamic headphones, as it does not deliver enough power for orthodynamics. Lyr 2 or Asgard 2 would be a better pairing if you want to use Audeze, MrSpeakers or Hifiman products (they are far more versatile).
At the end of the day, I vastly prefer the Valhalla 2 with both HD800 and HD650. You really won't find a better fully tube driven output transformerless amp until you seriously increase your budget by three to four times. However, I also own the Bottlehead Crack for two reasons. It is easier to listen to when I am tired and do not want to be attentive to my music (kind of hard to explain). And also, I love to DIY electronics (physicist by practice, work in a quantum computing lab, so electronics are kind of my life), and the Bottlehead Crack allows me to tweak parts as I see fit, and observe if there are any changes to the sound (usually only tubes and capacitors make a small but noticeable difference).