I went from E17, to E9 and my raod ended on the Asgard (i think 1). At this point i no longer wish to improve que quality by the Amp, maybe change the E17 for another DAC like the Bifrost, but it will have to wait till my wallet regenerates a bit.
Ohnoitztotoro When it comes to Volume, the HE-400 get good volume of an E17 (gain 12 volume 50+ that is), but at that point i belive some sound quality is lost.
I tried the E9, and while it was Loud, it felt a bit empty (dont know how else to describe it). However with the Asgard, the sound synergy really worked great.
I havent tried the amp you mention, but id suggest you check if you can try something like the asgard in a store to see if your amp is the reason you hear lower volumes, or if its the HP itself.
On a side note, as far as i know, if something sounds louder at the same volume (assuming same eficiency, etc), means the sound quality is worse.
But i belive it has nothing to do with that, probably the HD650 are easier to drive for your particular amplifier.