Voltage matching issue causing a hum?

I recently purchased a Yulong DAH1 Mark DAC. Most of you may not be familiar with it, but here is a link:

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEWNX:IT&item=120306659461

I use the USB input along with the "DAC OUT" jacks to go into an NAD T162 preamp, which then go out to 2 NAD C272 power amps. The sound is pretty good, but I notice more transparency when using the "LINE OUT" jacks on the Yulong to go directly to the C272 amps. However, this creates a VERY loud buzzing noise through the speakers, to the point where it's not even usable. I suspected it was a ground loop problem, so I used a power chord with the ground prong removed, but still hear a loud buzz. Could there be an issue with the voltages not being matched between the Yulong and the C272s? I also have a similar issue when hooking a Bellari VP129 phono pre directly into these amps.

Thanks