Reading NwAvGuy's thoughts on this subject (nwavguy.blogspot.com, I like his general approach) I came away with three main criteria. Feel free to correct or add to these if you think them mistaken.
1. Ideally, the headphone output should not exceed 5 ohms if it's to drive my 40-ohm cans properly. This rules out the headphone amps of most current soundcards, it would seem, and means that DACs such as the UCA-202 would also need a separate amp for best results.
2. It would be better to have a separate power supply, rather than have the USB doing both jobs.
3. If I want to use the Windows volume control as well as the amp volume knob (I quite like that flexibility), there's some value in a 24-bit connection even if the source material is 16-bit.
One obvious answer is the combined ODA+O2 device based on NwAvGuy's design sold by Epiphany Acoustics (http://epiphany-acoustics.co.uk/products-page/dacs/ehp-o2d-miniature-desktop-headphone-amplifier-with-usb-dac/). I have two reservations about this: (i) At nearly £200 ($300), it's a bit more than I ideally wanted to spend, and (ii) I have a feeling that it may be overkill for my headphones. I like the ATH-900s very much, but I believe they're fairly mid-range in the greater scheme of things, and I don't know whether they would show up the differences between this and something considerably more modest. Is this realistic?
A slightly cheaper alternative would be the Aune X1 Mk2, at about £140. Information about this is a bit thin on the ground, although the few opinions seem pretty positive. In particular, no figure on the output impedance, though they claim it is suitable for 30-600 ohm headphones. Anyone know more about it, or have similar devices they can recommend (available in Europe would be nice!)?
It seems difficult to find a DAC plus headphone amp combo, even in two boxes, that's noticeably cheaper overall than these, unless the amp is designed for portable use, in which case it relies on the USB for power, which I'm trying to avoid (e.g. UCA-202 plus Fiio E5, or just Fiio E10). Is this concern overrated? (edit, actually I've just noticed that the UCA+E5 combo doesn't do this, so is maybe a good answer, albeit not a very elegant one)
Lastly, I could just stick with what I have, which is serviceable enough, particularly if people think that none of these choices will make any difference to my listening experience, and wait for better alternatives to appear. In case it's not obvious, I'm only interested in measurable differences that show up as audible on double-blind testing. That's why I'm asking this question here, as these forums seem to have quite a few expert people who share this approach.
Thanks in advance!
Edited by thriddle - 4/30/13 at 4:23pm