Headphone Amp for Work

I work in a normal office environment--there are A/C units overhead, people in the cubes next to me talking, clicks from their mouses, etc. etc. This is not conducive to HiFi! :)

I eventually landed on the Audio Technica M-50 headphones for their combination of isolation (there is at least some), comfort (very comfy indeed), and decent sound quality (maybe a bit hyped in the bass, but the bass does extend lower and cleaner than I've ever heard in headphones, also a touch too much clarity relative to neutral).

I've been using a Presonus HP-4 headphone amp. It doesn't sound half bad, especially since I'm not going for audio nirvana here. The problem I have with it is that the volume pots show their price when used at low settings---which is where I have to set it not to melt my head. The left/right volume matching is pretty bad. I tried all 4 outputs, and use the one with the best matching at low volumes, but it is still annoying.

I'm driving it with a Griffin iMic. It's cheap. I wouldn't use it for my real system at home. But it beats the crap out of motherboard audio.

So my question to the gurus here is: what's a reasonably priced (i.e. < $200) headphone amp with a more consistent volume control and audio quality at or above the HP4? Contenders are Firestone Audio's Cute Beyond (which has a -20dB low(er) gain setting that might help keep the volume knob out of the poor matching region), Creek OBH-11 (one's listed on here for $129 right now), or.... ?

Thanks for any help.