Help with Adcom GTP-760 problem?

Hi, fellow 'philes and HT buffs. I recently installed a HT system for my ex-wife and her husband, and encountered a problem that has me stumped. The HT system has an Adcom GTP-760 pre/pro, which is connected to a Pioneer DV-37 DVD player. When the two units are connected using a digital co-ax cable, the system produces a noticeable, high-pitched whine that sounds like a distant jet turbine. This occurs only through the digital co-ax connections -- the TOSLINK is dead quiet. The whine remains at a steady level whether the overall volume is turned up or down, and it is reproduced through all 5 channels of the HT system. I tried using 2 different digital co-ax cables to eliminate the cable as the culprit, but the whine is present when using both cables (one from BetterCables.com, and the other is a Monster Cable Datalink 100). Any suggestions on what may be causing this problem, and a possible cure?

I have another question for owners/users of the Adcom GTP-760: how do you find the audio quality when reproducing music only (i.e., in use solely for audio, not HT)? My initial impression was that the sound is very clean, transparent, and dynamic, but the high frequencies seem to be a bit too emphasized. For example, cymbals, high notes on pianos and guitars, etc., seem a bit prominent, although cleanly reproduced. I'd be interested in the experience of others with this unit. Thanks.