I'm going out on a limb here because I've only read reviews on the Vali, not tried it myself. Also, although I'm curious about tube amps I admit being a fan of solid state gear so keep this in mind while reading the following.
According to reviews there are some issues with the Vali (check out some of the comments in the Vali thread) and -for a tube amp- it apparently sounds very neutral which in my book is not a disadvantage per se but if you want a 'typical' tube sound it might not be the best candidate.
Otoh I do own a pair of DT-880 in 660 Ohm and an Epiphany Acoustics O2D (UK flavour of the O2 amp + dad). It sounds extremely neutral and I found it to be very accurate, especially in its low gain setting, no distortion at all. On high gain and high volume it's not perfect though (distortion) but what gear is at this price level. Anyway, it manages to drive my DT's well, better in fact than the FiiO E17/E09K combo which I also own and which outputs more power than the O2. From personal experience I can recommend the O2.
The Beyers tend to run a bit thin and shrill in the treble to my ear, nothing to do with an underpowered amp as they also display this behaviour with my vastly more expensive V800/V200 combo, you should be aware of this. Very good cans for the rest -they need some burn in time btw, in the beginning they can sound positively shrill) but that seems to be the family's sound signature. If you don't want to accentuate this you should at the very least avoid a cold sounding DAC/AMP combo. My recommendation: get the O2. If you're set on tubes then try looking for an amp that'll give you a 'typical' more lush/warm tube sound.
