I would suggest having a listen to the Graham Slee Solo SRG2 or the Ultra Linear Edition of it. If you want something more rhythmical though less musical, try out the Vioelectric amps; I think you can get an on-board dac for the vioelectrics if you pay extra. I love both, I have both and I have a HD650, HD800, and a Grado sr325Is which are all great with the amps i suggested. Now, some might disagree but the ~100$ amps are simply rubbish with the 650 or the 800. They are not musical nor are they really "sufficient" for any of the phones. They lack a lot of sound and control with cheap amps like a Fiio. The Fiio E12 does have a lot of power for a little box but it is not sufficient and the sound is pretty bad too. You don't need the HD650 or the HD800 if you're thinking of buying a cheap amp. Maybe think of some Grados or something, which by the way sounds better with an excellent amp. The Graham Slee is, in my opinion, the best amp money can buy if you're thinking of a solid state. Burson amps are nice but they have too much sparkle in highs and treble for the HD650 especially because the 650 is supposed to be dark textured. Burson, I think is excellent for something like a Beyer. Beyerdynamic amps are also like Bursons, not much good for the HD series. Graham Slee has excellent bass, treble, and so on. HD650 and the HD800 will sing for you with a Slee amp. Also, I'm not a fan of tube amps so you'll have to ask someone else for that. I listened to WooAudio which is fine but not much of a fan of any since tube lacks the punch and bass control if you don't spend quite a bit. I also like things to be forever like a solid state should be. Hope this helps!