Scott,
Sorry I missed your call. The only one I ever used besides the one built into the PSR9000 was a rack-mount Yamaha unit. I don't remember the model, as it was several years ago. I don't remember any noise at all from it, and there isn't any in the 9000.
I'm sure you have double-checked all input and output levels, and are using good cables.
Other than that I can't offer much help.
DonM