Why not have it all? Why choose? I think it's nice to be able to have an amp that plays the HD800 true to its sound and one that alters it for a different presentation. I think the Vali actually allows more detail and an analytical approach for the HD800 over Crack. But it's also lacking in body and becomes more of a studio recording session than a relaxing, enjoy the music session. It all depends on what you want to achieve.
I'm in the cheer-squad for euphony and musical involvement. The HD650 is one of my favourite headphones and I often overlook its deficiencies in favour of engagement. The Vali is firmly in this category IMO. For all the praise, it is NOT the be-all. I have issue with its lack of control in the bass region compared to the Mjolnir. But it IS a very affordable amp with pros that greatly outweigh cons.
The clarity in the upper registers, the depth of soundstage, the rendition of low-level detail, the liquidity in its transitions and that pleasing reverb like quality - all combine to render a captivating analogue signature, sans the treble edge and glare evident in SS.
Relaxing and enjoyable? Yes. Analytical (in that dreary, sterile sense)? Far from it.
I've always been curious about the Crack's supposed synergy with Senn headphones. If it's a component that makes the Vali sound comparatively thin and analytical then it is likely one component that goes too far in colouring the transducer. Your posts serve as a warning for me to stay clear of it.