suggestions for a headphone amplifier

Another vote for the Antique Sound Labs Head MK III. This is a fine, economical intro to the magic of the SET experience. I thought the MF X-Can perfectly capable, but the ASL added that special quality of living air; with a marvelous hang-time for a note's resonant fade-out ...

Group,
It's one day later and I am savoring all of the suggestions.Some additional snooping on my own has uncovered Audio Valve RKV MK11, Singlepower offerings, enen a Gillmore new offering or the established Little w/ dedicated power supply. In some instances my 500.00 dollar ceiling is blasted, though I knew that would occur anyway. Headroom amps are for the most part unavailable for now, and I guess that their new line won't show for a month or two. Looks like there's a ton of great product out there. Tough choices!!!!!!

Don't forget to replace the stock 650 cord with the Zu Mobius one. I use a Headroom Max amp (with the new reference module that is in the latest version). This all works fine with me. Kubala Sosna Emotion ICs work well well with the 650s, as the ICs complement the 650's slightly dark sound.

There are several aftermarket cords for the Senn 650s. I like the Equinox cable. Others like the Zu but some say it's too bright. I tend to prefer a darker presentation.

Interesting comment that the Zu cable is bright, as I think the Cardas cable is bright & that the Zu is actually mellower & even a little darker than the stock 650 cord. I'll add my agreement that the 650 is a very significant improvement over the 600s, even with the stock cable.

I actually switched from the 600s, which I found to have a confining sound & distant presentation, to Grado RS1s (a very nice headphone with the optional "comfy" pads) until I tried the 650s.