I recently acquired one of these: To add a small amount of compression to my room mic feed, and couldn't resist running my guitar through it after one of these: That sounded pretty good for recordings, and got me wondering if I could add an effects loop to my Princeton Reverb and, if so, how to accomplish that without drilling holes etc. I wound up replacing the extra speaker jack with a switched stereo jack, soldering up a breakout stereo cable with 1/4'" plugs for I/O on the compressor and adding it to the circuit just prior to the phase inverter. I had to add a cap on the return side to block about 40VDC, and to my amazement it worked well off the bat with very little (if any) noise and sounded great. The switched jack allowed me to bypass the loop when the breakout cable is unplugged. Using the Princeton Reverb preamp (including reverb) rather than the PreSonus preamp (and AC adapter) makes it more gig-ready and adds a lot of flexibility. Just wanted to pass along my experience in case anyone else wants to use line-level effects rather than a pedal for whatever you'd like to do. I'm sure I'm not the first to do this, but I haven't noticed any similar posts for a while. My guess is it would be possible to add a similar mod to many amps.