I'm sure you know this, but ohms doesn't mean what you're saying it means. They're actually 18ohms and some unknown sensitivity, so theoretically, the shouldn't be the easiest to drive, since they need more power. You'll get volume yes, but not very much control from a DAP. Of course there are general "problems" to the sound when comparing them to the HD518. That's like comparing an apple to a frying pan and testing which one skis better. The Pros are meant to be for modern, poppy music. If you're comparing them to the semi-neutral HD518, you're doing it wrong. Really wrong. Nobody here compares the HD598 and PRO900, why would we do so in this situation?
See above. And uhh, I've heard about 5 different pairs because I refused to believe that they actually aren't that bad at what they're good at, and there really wasn't "so much" distortion.
Seriously, the bias here is disgusting. I'm not advocating that people buy these, and I'm not saying I love them (the only headphones I've "loved" are the HE500 and DX1000) but for what they are, they're not the worst things in the world. The Solos are where I draw the line, and I probably wouldn't ever buy a pair of Studios, but if given the chance, I'd definitely buy a pair of Pros at like $150.
A headphone doesn't need to distort that much for its own distortion to make whatever the amp is doing almost negligible. The Solo HDs are well above 1% THD over the entire spectrum. You're not going to be able to tell the difference between amps through those. The regular solos aren't far behind.