Regarding power and volume. Tyll's Innerfidelity measurements show that it only takes .02 mW to get 90dB out of the Momentum (same for the Amperior, btw). for comparison, the new Apple earpods need 10 times more power to hit 90 dB. The Momentum really do hit high volume with a tiny amount of power.
I think the reason we need to crank the volume slider up on the Momentum is because the drivers are placed noticeably farther away from the ear due to the thick pads.
So I wouldn't say they're power hungry (the data just doesn't back that up) but they do need some higher gain than what we get out of an iPhone/ipod simply due to the driver's distance away from the ear. Anyone who's played around with flat pads, L-cush and G-cush pads with Grados knows what a big effect these can have with perceived volume.
I'm curious to try these with a pico slim or leckerton UHA-6S but I don't know if the sound quality difference will be worth the inconvenience - I really do like having that inline volume and play/pause button.