No worries. The Momentums, though expensive for sure, can be found for much cheaper on sale. I managed to get mine for $300 from JB earlier. I'd order them off the 'net or from overseas from an authorised dealer where possible. The MSRP overseas is closer to $350, and in Australia it's 399 AUD from the Apple Store. I'd go overseas/online on them, Sennheiser's warranty is international.
I think (I don't really know amps) all cans benefit from amplification, but some, usually designed for portable use or as consumer headphones, don't require an amp to sound great. M-100 and Momentum fall into this category.