Looking for a USB Headphone amplifier

I am currently using a Music Fidelity OBH21 amp with a V-DAC to drive AKG 701 headphones. Though this system provides detail, I would like to improve on the ’soundstage’ and add a little more bass.

To this end, I am looking for USB headphone amplifier and also may upgrade from the 701s. Would like to purchase the amp 1st, one that would support a headphone upgrade.

Any recommendations?

OK, after much consideration I have decided to look for a used stand alone headphone amp and use a separate DAC. My thinking is that I will get more AMP for my $$$ and greater flexibility with this approach. My budget tops out ~ $600 for the amp (again used).

Any suggestions for an headphone amp that one feels is better than the Creek OBH-21 (original version) within my budget?
Have you looked at the PS Audio Sprout? It was just reduced to $500, which is within your posted price-point. It's an "all in one" that has both a headphone-out as well as speaker posts. I haven't tested one myself, but it has been receiving good reviews.

Hope that helps.
If you live in the US, give The Cable Company a call. They have a lot of headphone amps/dacs in your price range. If you take advantage of their demo program, they'll send you a box full of demo units so you can try them all at once. They also sell your Creek, and will be able to make some comparisons for you.
Courant, thanks for your reply. I am familiar with, and had considered, the sprout. However want a stand alone amp.

ZD, great idea, thanks! I should have thought of CC myself. I will give them a call.
Any thought on the quality of the Benchmark DAC-1 as a headphone amp? As compared to my OBH-21?