From Senn's website, 50 Ohms.

  Directly out of the headphone jack on the motherboard for gaming.   I also still use my 598s with my practice amp.   I won't make any claims about the sound.  I've virtually always used Sennheisers and I like them for anything.  I'm pretty sure after 100 hours you're going to prefer the...

Yo Yo Ma/Chris Thile - The Goat Rodeo Sessions Big Wreck - Albatross Black Sabbath - 13   Currently listening to Interpol - Turn on the Bright Lights   Next up is...The National - High Violet

I would have suggested the HD598s, but I have no experience with any of what you have listed as options.   When I got my first pair of Senn HD5xx something like 10 years ago it took a couple weeks until I was banned from all the east coast CS:S servers. That kind of sound imaging is as good as...

I like the idea of the E17, because even when you decide it needs to be replaced it's not useless.  Band it to your phone and have a nice portable setup.   I was in the situation and I bought the JDS Labs O2+ODAC.