Yep at the moment boiling down to the speedball + crack vs a valhalla 2 can't decide.
I've owned both (for extended periods of time) with the HD650 and HD800.
SB'ed Crack is the more (stereotypically) tubey, warm, bloomy, dynamic amp with nice slam/macrodynamics (OTL's tend to excle a there latter qualities). The Val2 is cleaner, faster, (much) more resolving, more transparent, neutral and boasts wider freq. bandwidth. The Val2 has a "solid-state" like sound but, IME, it has the ability to render microdynamics and detail more naturally than most solid state amps I've heard. There's a certain 'fluidity' and 'dimensionality' to the Val2 sound that I simply don't find in many solid state amps. The Val2 is often said to possess an 'Eddie Current'-like sound (which is a very high compliment cause EC tube amps are regarded as 'tubes done right'). My nitpicks with the Val2 is that it's a little 'dry' sounding, it lacks some slam, and the treble can be a little grainy (some
measured tube rolling may resolve this though). Due to the Val2's highly resolving nature, it's pretty sensitive to DACs, too (moreso than the CrackSB, which I wouldn't consider an
elite resolving amp.. even with cap mods, etc.). People have paired the Val2 with the Gumby (Schiit Gungnir Multibit) and said it scaled with that excellent DAC.. so consideration for upstream gear is warranted. Personally, I'd probably pair the Val2 with the Bifrost 4490 (or MB) w/ the HD650 and be pretty damn content until I was ready to spend $1K+. I had a souped up CrackSB w/ the HD650 a few years ago and loved it, as well.. extremely engaging, fun, and easy to kick back to. I could absolutely live with that combo, too.. it certainly possessed some qualities that are very hard to find elsewhere in its price range, let alone double it. There's a certain warmth and fullness to a carefully built/tweaked Crack that's simply captivating. Something both amps excel at is timbre.. the CrackSB is a little more saturated here.. but the Val2 is more accurate, but can come across as a little sterile (depending on your DAC/tubes, again).
So, it depends on what kind of sound you're looking for.. and what else is in your system. Personally, at this point in my journey, I'd probably opt for the Val2 (with good tubes) over the CrackSB cause I tend to value a more detailed, transparent sound with more finesse.. over persistent coloration and warmth.