Actually, highly sensitive, low impedance headphones like the 7520 are the hardest to drive well.
People often confuse getting a lot of volume out of an amplifier, with being easy to drive.
High impedance headphones mostly just need a lot of voltage to drive them loud, though ideally you would still be using them with a low impedance headphone output.
Low impedance headphones need a lot of current and a very low impedance amplifier - less than 1/8th the headphones themselves. And they need a very quiet source to avoid any background hiss.
Some reading on the subject: http://www.benchmarkmedia.com/discuss/feedback/newsletter/2011/12/2/0-ohm-headphone-amplifier-sonic-advantages-low-impedance-headphone-amp
As far as sealed headphones go, the MDR-7520 are my favorite by quite a bit. I think it's a shame they are so often overlooked, because they are such a good headphone.
Very true. I generally just use these with my Iphone but they scale really well and sound flat out amazing driven by my m903.