Actually there is a distinct difference in use the AUX port vs the USB option.
AUX uses the 3.5mm headphone jack which outputs an analog signal.
The USB option uses the dock connector which outputs a digital signal.
OP, what are you plugging into your AUX port? iPod or iPhone?
You can buy a dock connector cable that has a 3.5mm plug at the other end....this will work for both iPod and iPhone or iPad as well.
If you have a bluetooth device (iPhone or iPad) you can get a bluetooth AD2P streaming device such as the BlackBerry stereo gateway to stream music to the AUX port, using the digital output.
BTW, don't fall for the Monster hype.....it's all BS. Any decent quality wire will preform the same.
After running 50ft+ is where Monster products are useful.