There were some threads about the filtering at < 96 Khz input, to which the simple solution was to upsample at the source. But I see now that the audio-gd page shows a bunch of jumpers inside the unit, so I might try those.
My unit doesn't have the jumpers

I inquired of audio-gd, these were added in August 2011. The original unit (which I have) is effectively permanently set at: OS: 2, Filter: 3.
I've just doubled my "investment" in headphone DAC/amp, and am very pleased so far. We'll see if it lasts... The lesson I'm learning here is that the "hot" feature of the day (asynchronous USB, Sabre chip, whatever) means - in and of itself - not all that much. I guess it all comes back to implementation. Having said that, my new unit has a "32 bit DAC"...
I listened to another buzzword-compliant (but still relatively low cost) unit before buying this one, and by comparison I do think the GD is fairly neutral in tonal balance, just not the last word in clarity.