Ha, I too am tiring of my UEs. I'm actually doing a swap w/ friend today and he'll borrow my UE 5EB's and I'll borrow his RE0's. I'm also lending him my Ray Samuels Audio Hornet M portable amp and Shure 840's.
It sounds like you might want a closed pair of phones. Beyerdynamic and Shure 440/840's fit that bill. I've had the Beyer DT770 250 ohms but they were hard to drive properly, you need a good amp. Plus I didn't like the sound signature as much.
As I lived w/ my M22's, my ear for detail improved. I think that's why I don't like my UE's anymore. You should start by figuring out what music this will be intended for and what kind of signature you want. I found the Beyer's to be a little dark for me. I wanted something close to Axioms and folks here pointed me to Grados (fortunately). I LOVED the 325i's that I started with and now I have a pair of HF2's (limited run). The problem is that they are open so the sound bleeds in and out.
I'm kinda w/ kcarlie in the fact that I don't know if I could tell the diff between FLAC and ALAC. I can tell the diff between mid-range MP3 and ALAC though. I'm aware of the Redwine mod but I don't know if my money is best spent on that mod. I'd rather buy more music. Dr House makes a great point with Rockbox. If you really want FLAC/OGG/etc, get a cheaper iPod and hack it w/ Rockbox. People rave about that, though it does require some work on your part to hack it. OR, get a player that supports FLAC. Me, I just stuck w/ ALAC so it works on all my iPods/iPhones/shuffles/etc.
There's lots of reviews of headphone amps on headfi. Read them w/ a grain of salt, it does get kinda of 'AVS forum' over there. Personally, I've used a Leckerton UHA3 (great with a DAC feature), iBasso D10, and RSA Hornet. I have used a Little Dot I+ w/ modified op amps and 408A tubes and it was great with my Grado 325i. I liked it so much that I sucked it up and bought a Bottlehead on audiogon. I got rid of the UHA, though it was great. I really like the D10 because I can use it w/ the optical out on my Macbook. It is also a USB DAC that charges via USB, a GREAT portable feature. The RSA is also very good, an SS amp that has a bit of the 'tube' character to it. It's just a bit pricey, but luckily I bought it used on headfi.
Good luck.
