I have not heard the Crack (since I don't have HD600 or 650), but I know some guys who like it a lot and they are exactly the sort of listeners who would notice the relatively poor timbre of the Vali. I'm gathering that the majority of readers of a thread about $119 amps don't listen to music with much variety of instruments, so they would not notice.
Wow, that's a REALLY bold (and nonsensical) assumption to make! In what way is the cost of an amp or someone's budget at all indicative of the type of music they listen to?
I specifically said "majority" because it was simply a demographic analysis, not an assessment of any particular person.
Usually it is older people who have the money and interest in more expensive audio gear. These days, due to the economy, it is even more true - younger people are having even more difficulty than usual getting jobs, paying off student loans, etc.
And, for whatever reason, polls and sales show less interest in acoustic music amongst younger people than ever before. It seems possible that classical music and acoustic jazz music may both die out in the coming decades.
So, not referring to any particular person, but statistically, a thread on an inexpensive amp is going to get more younger head-fi'ers, and thus less acoustic music use.
I did listen to a bunch of EDM (older Daft Punk) last night uisng the Vali with the Mad Dog and my laptop's built-in DAC, and it sounded very enjoyable. I'm going to keep it for that late night use. The excellent detail and fast response of both the Vali and the Mad Dog makes the pairing ideal for modern electronic music.