I actually use my blackstar HT-5R as my headphone amp for my AKG Q701s now.
I have a cable going from my X-fi xtreme music soundcard into the guitar amp line in, and my headphones plugged into the headphone out on the amp. I just power the amp on and keep the standby switch off (which stops sound coming out the cab speaker). It sounds great, and it's is incredibly loud. There are no volume controls for the headphone out on the guitar amp, so I guess it has a constant level of amplification, but it is so loud I can't go above 40-50% volume on the PC without it becoming too much.
I should add though, that my phones do sound nicer when I plug them directly into the x-fi soundcard, it's just that I'm too lazy to switch the cables around several times a day!