250 watts RMS x 5, 20 Hz - 20 kHz, 8 Ω, all channels driven
input and driver stages pure Class A
output stage pure Class A first 8 watts ,
60 amperes peak per channel,
2.2 kVA toroidal transformer and 164,000uF power supply
built like a tank, all 80 lbs of it, but much prettier and designed by John Curl to boot! What's not to like?
It's laying around, and therefore easy to try, so put it to use (with 5 channels a bit over kill, the A21 would be more portable if you also have one of those). Even if your A51 weren't laying around, you should try it. I'd start out with the Audio Sensitivity Control set to manual, use the balanced inputs, and a passive preamp, if possible. For a full, 80 member symphony orchestra test where all the orchestra sections are easily discernible, get your hands on a copy of Reference Recordings HRx DVD-R 176.4 kHz / 24-bit release of Britten's The Young Person's Guide to the Orchestra, -- Stern/KCS (RR HRx-120). Adjust the volume so that the peak hits 0 dB during the last section; i.e., the molto allegro fugue. During the crescendo pay particular attention to the low end. If the amp doesn't clip, the AB transition doesn't become noticeable, the low end sounds don't become flattened or muddy, or the instruments conflated, the amp will handle anything you might listen to with the HE-6s. In fact, your search for an amp to drive your HE-6s may well be over before you start, room heating in the summer and electric bills aside. At the very least, you'll know what those of us who say "use speaker amps" are talking about.
Search this thread for advice on how to sequence turning on/off the signal chain. I start with the source, then preamp, then amp with the HE-6s already plugged into the speaker outputs and turn off in the reverse order. Some plug in their headphones after everything is turned on and unplug them before turning anything off. To minimize risk given this amp's power, I would emphasize as most important that you make sure the gain is set to zero before you turn anything on or off.
And, of course, by all means report back asap :)