Hey Rick, no worries, a more elaborate fix (fair bit of work though) is to do what Zsoli has done (http://adadepot.com/index.php?topic=311.15#lastPost) and replace all the opp amps, you'd probably need to get a tech to do this for you though, also not sure what it cost him in the end, but from what he says, it really cleans the unit up.
I guess ADA went for that loop pot setup to suit pedals with no in/out gain controls so as you increase the loop send it decreases the return etc. The idea would be adjust it to the point where the decimator has good input level (better signal to noise ratio) and turn the decimator output such that there is no increase/decrease in overall level when the loop is in compared to when it is out. I know Steve (RandallRG, and others) uses his 3TM loop so may have some good tips. I never used it when I had a MP1 and I know MJMP isn't overly fond of the MP1 loop as it can be tricky to set.
When I google the Decomator pro G it has 2 channels, if yours is like that you could put it between the MP1 and poweramp e.g. guitar > MP1> MP1 A out to Dec Chan 1 in, MP1 B out to Dec Chan 2 in, then Dec Chan 1/2 outs to poweramp A/B ins unless you just want in on specific patches.
The switch on each channel (+4db or -10db) is to set the decimator gain (input sensitivity, output level), so +4db for line level things, -10db for instrument level things. If you put it between the MP1 and poweramp set this to +4db, in the MP1 loop it will work either way but on the -10db setting you'd turn the loop send down, on +4db turn the loop send up.
Cheers Richard