Don't worry about which box you get, no one has actually compared the two as far as I know it is all very hazy rumor and I personally don't think there's anything to it. There could be of course, but it's not worth it I have a white box version and they sound the same as my last pair to me.
Also, don't worry about getting an amp before you get the phones. They sound good with just an mp3 player, so listen to that for awhile first, and then if you want to, get an amp later, that way you can hear the difference. I just don't think it's a good idea or necessary to buy everything at once. Better take your time, make sure the M50 is what you want, then if you want subtle improvements, get an amp for them.