No signal lock on my new used Adcom DAC

Just received an Adcom GDA 600 DAC with mods, which I purchased from a fellow Audiogoner. I have a good coax cable on order, but decided to go ahead and hook it up to my Denon DCM 370 changer with a Monster Interlink coax cable which I had on hand. The Digital Frequency Lock LED will not light up and not much sound comes through the right channel--in fact there is a lot of distortion coming through that channel. So, it appears that the DAC cannot lock a signal from my transport (the Denon DCM370 changer). Is this possibly the Monster coax cable? Or am I going to have a compatibility issue with the mods on the Adcom DAC? These include:

Existing dual Burr-Brown PCM63P 20 bit DACs piggybacked with high performance Burr-Brown PCM63P-K 20 bit DACs (4 DACs total).

Dual LC Audio AD825 OP AMPS installed.

Any help would be appreciated.