I've owned an A1 for some time now and I love it. The only cans it seems to have a problem with are HiFiMan He-500's. On the A1 I've powered Grado Ps-1000's, Beyer 770, 880, and 990's (the latter 2 at 600 ohms and the 770's 250 ohms, Senheisser Hd-800's, AT M50's and Denon 5000's. All except the He-500's are excellent. Mind you ... this is the only amp I've ever owned so I cannot make a comparison other than with the jack from my receiver but I love the sound. I have on order a Woo WA6se which will be here in about 3 weeks. I'm anxious to compare the 2 amps.
I have seen very little in the forums about the A1. It's too bad. It's a simple and very competent amp with a clean and detailed sound signature. It runs every can I own except the ortho he-500 with ease.