I've been very happy with my
Audio Technica ATH-M50 headphones, which I managed to find new on eBay for half the MSRP price by an authorized reseller. They're amazingly good, and their impedance is within the acceptable range for use with portable devices without an amplifier (though most audiophiles would recommend an amplifier anyway). They're highly durable with a very thick cable and a heavy metal plug with a spring stress guard to prevent fraying, and the only damage they've taken from significant abuse (including cold-weather/snow while walking during the winter) is a small fissure on the earpad, probably due to that cold-weather usage. The only problem is that the MSRP is $199 USD (the link above sells them for $139 USD, however).
Incidentally, they make great earmuffs.
Also, they perform very well with both heavy rock/metal and also with more subtle pieces along the classical genre. There's an awful lot of little detail that you can hear with these that you might have missed out on with lesser headphones (though I've never heard the Steelseries, so you may already have had a very decent set). They can get very loud without the audio breaking up if you're into that kind of thing, and they're generally just awesome. They've got powerful enough bass that they can actually be made to vibrate, but unless you crank the bass up manually, it's very well-balanced.
The only problem is that they aren't a headset (and nor are the Sennheisers ToeClaws mentioned). From what I hear,
Plantronics' Gamecom 777 is supposedly one of the better headsets available, both for gaming and for general casual audio. "5.1" is a little silly for headphones, though, but I hear nothing but good things about them.