[NuclearCrap]

I agree. I have both the DT880/250 and the M50 and I haven't heard anything that's as balanced as the DT880 for a similar price. Compared to the M50, the DT880 has less but quicker, more defined bass (quality > quantity). The M50 doesn't hold a candle to it when listening to acoustic instruments and/or vocal. I remember buying my DT880 from ProAudioStar about 4 months ago and they accepted an offer of $219. YMMV. I can't say anything about the AKG though since I've never tried those.

 

[Vonx]

Cutting the budget in half will get you a low-mid grade headphone and a low-mid grade amp at best.
 
@ OP, I would consider purchasing any of the high quality, low impedence headphones like the ah-d2000s, any of the beyers in the 32 ohm-80 ohm category. Then you can have a pristine listening experience much superior to having a poor amp and poor headphone. Headphone amps really only make sense for headphones that push impedence and listening to its boundaries and one that actually feeds voltage through would cost well above $200 which is not leaving much for headphones in that budget.
 
Start with high quality low impedence cans to have the foundation for a good listening system, then upgrade to an amp later on (because you know you will eventually)
 
Edit: Looks like you did. Dt880 250 ohm, couldnt have given a better recommendation myself. Fantastic choice. I think for them to really shine they should be amped especially since they are the 250 ohm and not the 80 or 32. 
 
I saw a FANTASTIC dac/amp/preamp with nothing but 5 star reviews and plenty of feedback, its the nuforce icon HDP. It retails around $400 bucks street price but it apparently has the quality of 1k to 15k amp set ups . It will REALLY do those cans justice and it would be the logical next step to foundate your inevitable addiction to audio that you will encompass as soon as you grab those dt880s and put them on.