External sound card for producing electronic music

I highly recommend the Focusrite Scarlett line. Particularly for you, the Scarlett 8i6 (http://www.focusrite.com/products/audio_interfaces/scarlett_8i6/) should be best. I own the said interface, and it's DAC was a clear upgrade over my former Asus Xonar Essence ST. The 8i6 also has four balanced TRS outs, a set of MIDI in/outs, two balanced TRS ins, a set of SPDIF in/outs, and two front-port XLR/TRS combo jacks. These front jacks each have fairly decent preamps.

The headphone out is not SUPERB (no sound to my DT990/600 ohm), but it gets the job done. It drives my my HD280s perfectly well, which shouldn't be much different from driving the HD25. The interface runs async USB 2.0 24/96, ASIO with drivers. You can also use it as a stand-alone DAC via SPDIF, if you have a SACD/CD/DVD player for example.

One final note, IT'S MADE ENTIRELY OUT OF METAL. Feels very solid, LEAGUES and LEAGUES above ANY M-Audio, Presonus, or Tascam product at the same price haha. Should run about $300 new, so it is at the upper end of your budget. 

What's the issue with M-Audio; it'll run ProTools (which is fairly standardized), and do what you want...

If you're doing actual production, don't even bother with USB - the latency is too high and it simply can't handle a whole mess of channels. You want FireWire (and that's basically the sole reason it still exists). 

Here's what I'd suggest:

http://www.m-audio.com/products/en_us/ProFire610.html

Or one of the PreSonus boxen.

If you lack FireWire (which is a shame), I'd probably just get an E-MU 0404 and be done with it. It'll save you some cash, and work for monitoring, and give you a few inputs. 

If you're really "serious" about this, you'll need either a monster FireWire external, or (more ideally) an internal; M-Audio and Digidesign are again what I'm going to preference. The Delta 1010 may be ancient, but it would work very well; the ProFire 2626 is the "new and hip" kid on the block (And if you don't mind all of the extra money for ADAT I/O panels, a real monster - then again, remember that you can link Delta 1010's up to 40x40 (just not in ProTools) and that the 1010 is line I/O). If you need to go bigger start looking at a real control surface. 

I doubt you really need that much I/O for electronic music though, and you're probably just wanting to monitor and maybe hook up a mic or keyboard or something, which the 0404 or some other relatively inexpensive device can get you quite cleanly (and let you make equally great jams). You might look at one of the Behringer or Mackie controllers to make things "neater" when you're mixing, neither is very expensive (at least not C|24 or MC-5 expensive).