After reading the Headfonia review, particularly the blurb at the end about the impedance curve effectively becoming the frequency response under Current mode, I'm starting to get a clearler picture on how the Bakoon drives different types of headphones.
In a recent PM exchange with a fellow Head-fier, I mentioned how I thought the HPA-21 was a very "reactive" amp when used in Current mode on dynamic driver headphones. On the ortho Audeze LCD-3 I had at the time, the Bakoon amped it in a very even, linear fashion. It didn't really change any part of the LCD-3's natural EQ in most cases. It just added detail everywhere and accentuated certain technicalities. In that sense, you could call it a predictable kind of amplification since orthos have near ruler-flat impedance curves, though the way the LCD-3 presented everything was still pretty stunning, particularly the depth and lushness at the upper mids.
The Fostex TH-900 however, responded quite differently. The mids gots raised up a bit more (though still a tad recessed, which I actually prefer), the upper bass came even more alive, and the treble got smoothed out quite nicely. It was a bit mind-melting, as I'd never heard any amp - I've tried a few (Burson, Violectric, Luxman, Fostex, Cavalli) - work the TH900 in that particular fashion.
I'm not the most technically minded Head-fier (nor do I want to be), but I checked out InnerFidelity's charts for the TH-900 just now; http://www.innerfidelity.com/images/FostexTH900.pdf
Look at the bright-magenta curve in the Electrical Impedance and Phase section. To my unrefined ears, that's almost how the Fostex sounded, or rather, how it reacted towards the Bakoon's Current mode. If anything, I'd say the resulting EQ was sort of an "average" between the Frequence Response and Impedance curves, if that makes any sense.
I never had the chance to try a HD800 on the HPA-21, but if it's impedance curve is anything to go by, results might be interesting; http://www.innerfidelity.com/images/SennheiserHD800.pdf
I kind of agree with Headfonia's conclusion, in that the HPA-21is not an end-game amp for every single headphone out there. For certain rigs and certain tastes, it could well be the best ever, but the way it reacts to different drivers/headphones seems to be quite unpredictable.
Just my uneducated 2 cents. Not worth much, but I reckon it might at least point the more informed folk in some direction.
Edited by kurochin - 11/18/13 at 5:44pm