How crucial is the quality of a digital cable?

I recently purchased a Cambridge Audio CX 81 integrated amp. The analog inputs sound great, but I find the internal DAC to be a disappointment. It uses the ESS Sabre ES9016K2M chip. I use a mid-level Rotel RCD 951 CD deck as a transport via the digital coax. I also hooked the deck via the analog out, just to compare. The analog interconnects are entry level AQ's. The digital coax is an Amazon purchase in the $15 range. I find the internal DAC of the 20 year old Rotel to be superior to the new AX 81, when doing an A/B.

What then confuses me is that the Blue Tooth streaming from the amp is quite good. It is HD aptX,  24-bit/84kHz capable. But, wouldn't that also go through the same internal DAC? Could it be the mediocre Amazon digital coax cable? I always assumed that digital cable quality was less significant than analog cables. Just bits and bites, right?

If someone knows of a decent digital coax for under $50, I would be very grateful. I would also appreciate any advice on an entry level power cord for the amp. Can a decent power cord be had for under $100?