Need USB cable to volume control to 1USB 1 Pr.RCA

Really like the Logitech Z 10 computer speakers which have great looks and (supposedly) decent sound but would also like to also feed a subwoofer via RCA's.Guess I could ask if you know if their is a USB to RCA set (though I could use the computer 1/8" jacks)but that would mean lowering and raising volume via computer much more cumbersome than a single USB out that went to master volume and then split singnal (intact so it could feedback track control as when used alone) and send other signal to sub.It's on the Logitech Z2300 which is same $150 has the wired volume set up that many PC sub sats have.But not having a variable line out was in my mind a definite oversight in such a nice looking product.Idea's??
Thanks
Chazzbo