I really hate to shoot you down like this, but I think you are wasting your time and money. There is so much more to USB signal integrity than the voltage at which the high and low states are at the end of your cable. You need to look into signal quality measurements to really get any idea of what you are seeing and if it might affect the signal integrity. For one, I suspect you won't even be seeing anything relevant at the end of your cable before your computer and USB-device have connected. If you do, you won't have terminated D+/D- lines which will make the signals irrelevant to look at anyway.
If you truly want to look at only the signal levels. You should break open your headsets USB-dongle and measure the voltages at the USB jack of the device. That will at least show you what the signals look like with correct termination and load. If your scope can capture and eye diagram, that's when you start getting somewhere! :)
Best of luck!