From those I've had the pleasure of listening to, it would be the Burson Soloist/Conductor. The current on these is fantastic - the K701/Q701 actually sounds more like a Grado (without the too strong L-C-R with recessed areas between L-C and C-R) and actually made them sound a bit better than my HD600. I have a Meier Cantate.2 and while it works great with the K701/2, it didn't have the dynamics of the Burson on these headphones. The HD600/650 don't improve much better on the Bursons though, but of course it's easier to find an amp that can drive them well. Has better current on the HD800 too but not as drastic as the change with the AKGs (and the Meier amp actually sounds great on those AKGs).