I love my AKG K612 Pro, it produces wonderfully detailed, spacious sound but after trying out different amps, I cannot help but resign to the fact that I cannot use it for other than some chamber music or acoustic guitar tracks, which I only have a couple of. So I very rarely use it.
The Vali is supposed to provide lots of power. Alas, it seems not enough for the AKG.
P.S. The Heed CanAmp is said to drive AKG 7xx with some good results in the bass department and although it is manufactured in my country, it is too expensive for me to pair it with my K612.