Hello,
Well, in just about every area. So, my primary basis of comparison is the Benchmark DAC2 HGC, which is the dac I use in my main system. Headphones I use are the Sennheiser HD650 or the AKG K701.
Admittedly, the DAC2 is almost twice the price of the HA-1, and that may be the issue and the real answer to your question...just how much sonics can you pack into a dac/amp combo for $1100..?
Anyway, through the DAC2 the sound emerges from an entirely black (noiseless) backround. I think this is a key factor for good headphone listening. The sound is very liquid and is reproduced with large, round, full tones and all the overtones. It's balanced from top to bottom. Spacial queues are captured too, and the soundstage is wide. Dynamics are quite stunning with great bass slam when it's in the recording. Regrettably, I just don't hear any of this through the HA-1.
My second basis of comparison is my neighbor's xmas present to himself - a PrimaLuna Dialogue Premium HP, which I'm baby sitting this week while he and his wife are up skiing. HP apparently stands for high-power or head-phones...take your pick. He's already rolled-in nos RCA 5814A tubes, replacing the front-end Chinese 12AU7's. I have to say that the headphone output on this thing is simply amazing and better than the DAC2. Everything I said above plus all the warmth and sparkle you can imagine. Staging is deep and huge, and the sound is, well, huge!
So, If Dan can pull this kind of a tube mod off for the HA-1, I'll be the first customer in line. Hope this helped answer your question...Denny