It really really depends on how loud you want your music to be, If you listen to moderate volume, the Momentums definitely have a very good, hi-fi sound even unamped, and while the the volume is better than any other Sennheiser I own, personally, I still think it can use more volume unamped.
However, even with the most modest of amps, the Momentum will truly sound (and look like) the $350 headphones that they are. If price/budget is an issue, I'd but the Momentums, break em in, get acclimated with the sound signature, all while you save up a couple of bucks to get an amp (like the following month). I dont think you get all of your money's worth without an amp.
The good thing though is that the volume is perfectly OK and the SQ is great even unamped (I dunno what kind of DAP you have, maybe others players like Ipods, Sansa Clips,etc have a bit more juice than the J3), and you dont need a super expensive amp to bring these up to another level and really get your money's worth (like I said, the FiiO E7 was more than up to the task.
These are some current prices for amps that should do the trick:
Fiio E7: $88
Fiio E11: $58
Fiio E6: $28
All are portable and the E7 is also a DAC for listening on a laptop with a weak soundcard. So my recommendation would be in favor of the Momentums, and just save up for a lil amp. By the time you get your amp, your Momentums should be fully broken in
