My Current Setup:
Currently I am using the onboard audio from my motherboard (Asus Maximus V Formula NO THUNDERFX).
This is usually hooked up to my speakers (Logitech X-540) as well as my headphones (Sennheiser HD280 Pro's). I hook up the headphones to the front panel usually, and never through the speakers.
What I am looking for:
Right now I am looking at buying a pair of Sennheiser HD598's as I listen to a very wide variety of music and they seem to have a rather neutral response (as do most Sennheiser's, if I am reading correctly), and because they are pretty efficient and low impedance (quoted at 50 Ohms, iirc). This means that I don't NEED an amp (yes, it would be nice to have one, anyway, but it isn't necessary) which makes my budget much more palatable.
I don't only listen to music, though, I also play some guitar and do a little singing, so I am looking at a USB DAC that also has input (I guess it would be called an ADC?), which severely narrows my options (if I look for a combo device which I am because I would imagine it is cheaper). Right now I am looking at the M-Audio Fast Track Pro. This is a nice device because it has phantom power for a nice mic, guitar input, and XLR input and output.
I am also considering buying a cheap(er) amp to use with this setup and to take with me to use with my phone (since using an Android phone seems to more or less preclude using a DAC I'm looking more for a standalone amp). The Fiio E11 seems to be a good choice in this area.
What I listen to:
I listen to a very wide variety of music but I would peg most of my interest in the rock/alt rock category. I also listen to a good bit of country and pop, a little bit of electronic (D&B, Trance, and House. Mostly anything other than Dubstep, although there is some of that, too xD), and a lot of classical and musicals.
I would like any suggestions people have for parts for the setup, whether it's a new DAC/ADC combo, a standalone DAC with an ADC, different headphones, grab an amp, whatever. My total budget is around $500, but I could be persuaded to move up a little bit, it would just take me longer to get the money :)
Other things I've looked at:
I've also looked at Beyerdynamic DT880's (which would pretty much require an amp) and HD600's (which are a little high for my price range).
As for amps/DACs/etc. I've looked at quite a few but I'm not very sure how to choose them. They all seem to be pretty much out of my price range or to perform similarly and I can't seem to differentiate between them.
Any help you guys have, words of wisdom, suggestions, etc. are very, very much appreciated. Thank you all for looking and helping me out. From what I've seen this is a really great community and I love it here so far.