Well, I've got a used SVT-4 and I've been trying to use my dbx 166A with it, and I cannot get it to work whatsoever in the FX loop. I'm using TRS-XLR cables, and I put the send to the dbx input, then output from the dbx output to the FX Return B (Mono), and I get...nothing. No compression. It's like the loop is stuck in bypass, no matter where the switch on the front is. There's no footswitch connected, either. I've tried using unbalanced connections, and the compressor sees no audio. With TRS, it sees an input signal, but the amp does not use the FX loop. I've read through the manual time and time again, and I'm stumped. I connect from preamp output to power input using the compressor in the middle, and there's no signal through there either. These are brand new cables, and the compressor is functioning fine. Does anyone have any idea what's going on?