So Bruce made a special run of this little Loop Level Matcher device, designed to bring line level effects send down to pedal (instrument) level, and then transparently bring it back up to line level for the loop return. What a great little device. I got mine on Friday. Today, after bringing my 2204 home from my church, I decided to see if it would work to turn my little passive power amp insert point into a full fledged active, buffered effects loop. I took my insert point and moved it to after the master volume pot, and viola! This little device gives me a great, transparent effects loop on my '79 JMP, without having to drill any extra holes! (I used the second speaker jack hole to mount a TRS jack)
So far, I haven't been able to tell a difference in tone or volume (after adjusting the loop send and return level pots) at all. I'm a happy camper.