I'm not sure what the deal with the x7 and x10 is, I've used the S4s for years and I recently warrantied them and exchanged them for a pair of x7s. I really enjoy the slick ceramic head on the x7s and the flat cable, prevents tangling really well and those ceramic heads are beautiful.
Overall I'd say take the x10s, if only because they're newer and the heads are nicer than the S4s. I busted open the head on one pair of the S4s, I think the better heads are for sure worth it. I didn't notice a sound quality improvement between the S4s and the X7s though, despite the fact that they're apparently worth $250 more.
Overall I'd say look into the X7s too, but if you're not interested pick up the X10s.
Buy them at a big retail store like future shop if you can, they've been really good about the warranty with me. I've been using Klipsch headphones for about 3 years now and I've gone through 4 different pairs, never had a problem exchanging them and the warranty renews with each new pair you get. Just make sure you keep your receipt!
