USB Isolator

I have a Yamaha DTXtremeIIISP edrum kit which I use to record midi (via USB) for later re-rendering using BFD2. While tracking, we monitor audio from the DTXIII brain. The USB midi from the DTXIII is USB 1.1, with cable length limited to 3 meters (per the Yamaha manual). The 3 meter cable limitation was a problem for me since the edrums are a ~18 ft cable rum from my DAW PC to the DTXIII brain. So, when I began using this kit, back in 2009, I purchased an IOGEAR GUE216 USB 2.0 Booster Extension Cable (16 ft). I plugged the GUE216 into my computer (the PC provides USB power to the booster).  I then plugged a 4 ft USB A to B cable into the GUE216 to run the remainder of the distance to my DTXIII brain. This all worked fine for midi via USB. Also, back in 2009, I noticed a small amount of hum in the audio from the DTXIII (unbalanced stereo audio outputs). I resolved this by inserting an ART DTI Dual Transformer Isolator, feeding balanced audio cables into my FF800. Jump forward in time to 2013, when I upgraded all of my DAW equipment, including the addition of an RME FF-UFX. My standards regarding noise floor on everything have gotten more strict. I became aware that there was still some background noise on the DTXIII channels into my UFX. This mainly consisted of a very low-level ~8 KHz squeal, which seemed to be related to the USB activity. I am now looking for a way to isolate the USB power/ground connection from the DAW PC to the DTXIII edrums to see if this eliminates the ground loop. Has anyone ever used a 'USB isolator' for this purpose? If so, was it successful?