Just to clarify as there are a couple of misunderstandings from a couple of the guys that replied to this post. I will hit RESET on the discussion and try to start anew and in brief. The delay unit that I was discussing in the original post is an ElectroHarmonx Deluxe Memory Boy (DMB). I bought it new off-the-shelf and did not have any mods done to it. I started off by hooking the effects loop send/return on the DMB to the send/return on the effects loop of my 5:50+. That was an error as the effects loop on the pedal is meant for putting other pedals into its effects loop, and not meant to hook up into the amp’s loop.
I then connected the DMB correctly to the 5:50+ effects loop by simply using the DMB’s input and output. The sound was initially poor, but after much tweaking I could get an OK sound, but not great. The repeats are somewhat distorted and muddy as compared to the dry sound. I attribute this to a possible impedance mismatch at the input, and also a possible problem with the gain in the pedal’s delay circuit. Reasons for those best guesses in a minute.
In parallel with this experiment I was assembling a Pedaltrain for my church rig (Side Note: I have a church gig on Sundays in Hong Kong and a night club gig in Suzhou China as regular guest player with a great rock band over there. All of this in addition to my paying gig as manufacturing engineer). Inside my Pedaltrain rig I have a Deluxe Memory Man XO (DMM XO) that I purchased new from Analog Man. For those of you who are not familiar with Analog Mike, he makes his own high-end pedals and also buys classic pedals that he modifies. He is an expert at doing mods to classic analog pedals. For my pedal, he did an input impedance mod, and an internal gain mod to clean up the repeats so that they sound nearly as clean as the dry signal. This pedal was not cheap when all was said and done; total around $550 USD. But well worth it.
I tested the DMM XO in the 5:50+ effects loop and it sounds awesome straight away. Much more musical sounding than the DMB, by far. The sound is clear and clean, which hints that the input impedance of the pedal is probably matched up OK. The repeats are really clean, telling me that the internal gain mod to the delay circuit was a good idea as well.
I hope that this explains things well enough, and if not I am open to questions and comments.
Cheers!