Interesting. Did you post some Classic vs Concerto impressions somewhere ?
I plugged both to my DAC and did some comparisons using my HE-500 and HD-650.
Sound: Very similar. The Concerto a touch brighter / Classic a hair warmer. But otherwise very similar.
Power: Classic has slightly more power. On the Concerto I usually require high gain to drive the HE-500, but on the Classic, low gain is usually enough.
Pairing: I do prefer the Classic for the HE-500, however I have a feeling that the Concerto is a touch better for the HD-650. But the difference is small enough that it can possibly be Placebo.
Features: Perhaps the biggest difference between the two, at least on paper. The Concerto is very basic, with 1 input, 2 gain settings, and 2 crossfeed settings (on/off). The Classic has got 2 inputs, 2 gain, 3 crossfeed settings (high/low/off), and 3 "bass compensator" settings (high/low/off). The 2nd input is excellent to have on the Classic. The crossfeed on the Concerto to my ears is closer to the crossfeed high on the Classic. With the Classic I rarely use the crossfeed low section, and mostly go between high and off, depends on the track. The bass boost is redundant to my ears, as it seems to muddy the sound for a mild bass boost. As soon as I switch it I immediately "run back" to the off position to restore the wonderful natural clarity it has.
Build quality: Mostly the same, however the volume dial on the Classic is plain plastic, and the dial on the Concerto is much nicer to the touch. However, the volume change is much smoother on the Classic - feels more like a continuum compared with the clear steps on the Concerto.
So, in short, the extra features on the Classic are not really a big improvement on the Concerto, as I never use the bass boost because it degrades the sq, and I also never use the low crossfeed section. However, it is nice to have the 2nd input and the higher power might come in handy for some particularly hungry cans. But the Concerto drives the HE-500 just as competently, really.
