You probably need at the least a line driver on the Empeg board and possibly a receiver to recover the clock signal. A 74HCT244 should do the trick. This could be used to buffer some of the other signals too. Maybe just shortening the clock cable to the absolute minimum would do the trick. I assume the input capacitance of the CS8405A is about 8pF or so maybe.

You could also AC couple the clock signal and bias it on the receiving end with a couple high value resistors to 1.4V. This may not work well though as I don't think the input is a schmitt trigger type.

Speedy