It might not be a warranty fail, but I won't be happy if they built me a gaming computer without enough power to operate a USB headset!
At any rate, I'll take it back next week and they'll check it out. If they don't find anything wrong, I'll probably try that Y-cable. Thanks for the suggestion.
ASUS and Plantronics are both well-known brands, though, so I'd think there'd be a lot more people with this problem if they just don't work well together.
Thanks again.