FWIW, a friend of mine tried all the alo amps and ending up buying the national for best synergy. But then she loves her bass- and it doesn't sound like you're as picky about bass.
So far the best synergy I've found is out of the headphone out of my hm901, single ended.
If you think that then you are wrong. I LOVE Bass but only tight, unbloated, non-boomy bass with visceral impact. Some would call what I'm describing is smooth bass.
An example would be the JVC DX1000, where the bass is plentiful but yet it is smooth and doesn't steal the limelight frm the mids, making it really very enjoyable. However, only a few cans around here manages to achieve that feat and I haven't seen this in earphones yet.