Hey Kevin,
Yes, I'm sure this question has been answered in 1000 different ways on the Forum here, but we don't mind. That's what we do here. We love talking about gear and different ways to set it up.
For the original MP-1, I've run my stereo effects processors right from the preamp outputs of the MP-1. You can set the output level from the knob on the front panel of the MP-1, or you can use the input level controls on the processors. (At least my Eventide and Lexicon processors have input level controls). In my opinion, this works out great because your Chorus is coming out of the MP-1 already split, and now, you're running the Chorus in a 2 channel stereo field, where the effect really shines. I would only use mono effects in the MP-1 loop because if I am not mistaken, the MP-1 will split that into two channels when the Chorus is engaged. So it's like putting your Stereo Chorus pedal very late in the chain. Personally, I prefer my delays to be after the Chorus effect. That way, the guitars still have the thickness that the Chorus gives it, and I can use different delays in each channel to manually create a ping-pong delay effect when I want it. I was doing that way before I got the Eventide or Lexicon gear.
Speaking of effects loops, do you realize that the tone you are hearing when your MP-1 is plugged into the effects loop of your Marshall 2525, is the Marshall preamp feeding into the front of the MP-1 preamp? You may want to keep an eye on the input LED of the MP-1 so you don't overload it on the front end. With an amp like the 2525, I would just plug the mono output of the MP-1 into the Return jack of the Marshall. That way, you are just using the power amp of the Marshall and the speakers, the Marshall preamp is bypassed. I've never run any of my ADA preamps into a Marshall before, but I have run all of then into the inputs of my old Fender Twins. They are loud and clean amplifiers that are all tube and they do take the ADA's very well. I used to use four of them, but now I'm down to two.
Of course, I still have dedicated power amps and ADA Split-Stack Speaker Cabs too. (I know. I have too much gear and I still get G.A.S.).
Harley