[High_Q]

I have verified it, tube is not for AD700.  Why is that?  Anybody know?

 

[Ra97oR]

Well, most headphones tube amps are design mainly as a voltage amplifier and tends to lack current that a low impedance headphones need. AD700 is bright and detail, where as most tube amp tend to distort and sounding warm. By using AD700 on a tube amp, either underdriving them (not really that possible) or the sound signature of the amp cancelling the good point of the AD700, making them sound meh.

Of course there are always exceptions, so a different tube amp might as well perform differently.