RH450 Screeching noise then no sound

Here's my setup: Saffire Pro 26: Input (firewire) from PC sound card, Output: SPDIF through DAC to RH450 Aux In. After playing various basses (J-Bass, StingRay 4 & 5, Ibanez) through the amp with only headphone output for a few minutes I get a loud screeching noise followed by no sound at all from the amp. If I connect a different cable it works for a few more minutes before doing the same thing again. The cables are all good quality (George L, Spectraflex, Monster).
The same cables and headphones connected directly to the Saffire work fine. Am I missing something about the correct way to connect a PC sound card through the Saffire to the RH450? I am inserting the Saffire into the mix because the max output from the PC alone just isn't loud enough.
I appreciate any help you can provide.