Yes, your Logitech USB speakers have their own audio decoder and completely avoid the soundcard(s).
It looks like there only Vista Drivers
for it, hopefully they work in 7.
In Vista all you need to do to switch between outputs is to go into the Sound control panel and select it as the default device, should be pretty similar in 7.
I do this in Vista for switching between an old X-Fi (surround speakers) and a Xonar Essence STX (K701's), works like a charm.
A shortcut on your desktop will speed up the process...