Don't bother with a pre-amp. Not needed, unless you get a turntable and need a preamp with an RIAA equalizer. Other than that, don't worry about it.
The first thing I'd think about (if you REALLY want to move into higher end stuff) is a good dac, preferably one with balanced outputs. This will pay immediate dividends to your current system. I recently built a y1 DAC, which is a high-quality but cheap single-ended DAC designed by member amb, and felt that that was the most dramatic upgrade I had made to date (besides moving into higher-fidelity headphones). This would be great for you, but is kind of a dead-end as far as moving up the chain. Buying something more expensive with balanced outs (The Benchmark DAC1 usually gets pretty good reviews here) would leave tremendous room for upgrading to a balanced amp to drive re-terminated HD600s or 650s. This is a TREMENDOUS system for *cheaper* headphones.
The other end of the spectrum is more expensive headphones... but be prepared to keep on spending if you go that route. HD800s, PS-1000s, Stax Omegas, second hand R10s and Qualias, etc. are great cans, but you'll want a source at least of the DAC1s caliber when you get headphones like that, and you'll end up buying or at least trying multi-thousand dollar headphone amps as well. I'd really invest in a better source at this point; again, you'll see dividends with your current gear, and you'll be more *ready* to move up the headphone chain and get good results if you so choose.
