Modifiying CD player to accept external I2S signal

In case you haven't already restored or cannibalized the Rotel, try a twisted pair for your MCLK signal (find a nearby ground at each end). Whether it is 11, or 16MHz, 74HC edge rates - or anything like it - are too fast to have a loose wire waving around, with its ground return path all the way diagonally across the board.

But suspect rfbrw's conclusion (post #39) is the real source of your troubles.

This is a Really Ambitious undertaking without a 'scope. Best luck.

Regards