I'd like to share my experience again as I have already mentioned in one of the DAC threads here before.
My HRT Musicstreamer II DAC had a significantly improved sound quality after changing the CDR-King USB cable I was using to an Analysis Plus USB cable I loaned from my brother. The music sounded cleaner, less harsh, livelier and had more bottom end. It turns out that cheap, poorly made cables tend to degrade the sound quality of USB-powered DACS most likely due to the quality of the power wires and the soldering workmanship of the cable which might cause voltage drops and the like. All the USB powered DACs I've owned have been sensitive to the quality of the power source and I confirmed this by doing loop-back FR measurements using RTA in order to make a calibration profile for the DAC. The low quality cable introduced a slight sawtoothing in the frequency curve where the higher quality usb cables did not. Additionally, removing the APC Uninterrupted Power Supply which had a simulated sinewave output to the computer's Seasonic X-660 power supply unit also further improved the sound quality of the DAC as confirmed with a simple blind test with the help of a friend. Upon googling the issue, turns out that simulated sine wave UPS's aren't designed to be used with high efficiency computer PSU's with built-in line conditioning and active PFC and it seemed like the 5 volt rails to the USB ports in the motherboard were affected.
I eventually got myself a Wireworld Ultraviolet USB cable since the Analysis Plus USB cable was no longer available from the supplier and got similar results. 1's and 0's aside, I can't stress enough that a good quality cable is important for usb-powered DACs because of power delivery issues. Doesn't have to be expensive, as long as the quality is good IMO.
