Recommend me an amp I can use with USB/Firewire

How much do you want to spend, $500, $1,500, $5,000? You can get some decent rigs for under a grand or spend over $5k for something real special.

It's hard but it's possible. I would take a look at the PS Audio GHCA, it looks like a serious headamp and has a USB DAC as well. But you won't have any $ left for headphones. A Sennheiser HD650 and a HiFiMAN EF2 DAC/amp could be had for under $600 though, you could upgrade the DAC/amp later. There's a lot of options out there, you could try a DacPort and some efficient headphones like Grados or Audio-Technicas. I found that the Maverick D1 synergises well with the Q/K701, that would be close to your price range and be a pretty versatile setup. The Q/K701 also reportedly works well with the Fiio E7/E9 DAC/amp combo and would offer a different kind of versatility but I haven't tried it. None of these options have Firewire capability, for that try the cheap pro audio options like M-Audio's Firewire Solo but you may need to install a driver. There's a lot of used gear in your price range from those that trade up, so you can stretch your $ that way if you want.

Bang for Buck:  Zero pre/amp/DAC off fleabay for $120-ish plus Sennheiser HD600 for $315.  Go digital into the Zero.  This will get you caviar sound for halibut price.

The HD600 also sounds great straight out of the Matrix Mini-i, that wouldn't be to far over budget and would be within it if bought used.