how long is your warranty? you have a couple of choices here:
1. it is in warranty still. send it off to logitech to fix, hopefully at no expense to yourself (it all depends on how good or bad the warranty agreement is... if they're like Koss headphones, you should be very happy)
2. it is out of warranty - this means you can't void the warranty by doing a little DIY on it... open it up and see if you can fix it yourself

of course, DIY is probably not the best solution - whah kind of distorted is it? is it louder than the other channels by a large amount? is it softer? is it noise from hard clipping? these things will help determine which bit has gone b0rk in your speaker.
from the sound of it one of the amplifiers has gone. i'd be inclined to send it to logitech, as i don't have the dexterity required to do repairs on electronics myself (i'm all thumbs when you put a soldering iron in my hand).