Hey gb, as you get a good understanding of all the different levels and gain structure of each unit (as they all have different roles/jobs to play) you then can combine the accordingly. There's a bit of mix and match involved with all the things you mentioned, so you have to understand each one so you can make sensible gain structure decisions about them (this goes with that at Susan's (you'll understand that quip no doubt
)).. And your questions and the various answers are helping you get your head around it all
.
The Lexicon instructions re "use send" are based on using standard amp head (predominantly). The send signal is typically line level but can often be set to inst (for stomp pedals) or line. The Lex input can probably handle either level (hence input vol on lex to adjust so it has plenty of level to work with (and thus less noise compared to signal)).
I'm a little confused though, if you run 4 CM (ie chain through Fx to poweramp) then that means Guit > MP-1 input, MP-1 A/B outs > Lex L/R in, Lex L/R out > poweramp L/R in > 2 x cabs (or stereo cab), this is what the 4 cable method is (I call it chain through Fx). So you then balance your MP-1 out level with the Lex input (so plenty of signal at loudest patch but no clip (you DON'T want to clip Lex inputs, digital distortion is not pleasant)). Then as you said, use the Lex output control to drive poweramp. I'd run the Lex out as hi as poss without clipping the poweramp (again better signal to noise) and adjust overall vol with poweramp vols (in this particular set up scenario). An obviously use the wet/dry mix level in the Lex to balance how much Fx you want.
MP-1 Fx send level is adjustable between inst and line via the dual gang reverse level pot, as you turn it up the send level increases and the return level decreases (and vis versa), I'm not a fan of the MP-1 loop, it's hard to get right, it's designed to be usable for inst level stomp boxes OR line level Fx units (mixing and matching stomp boxes and line devices here is NOT a good idea).
MP-1 A/B outs are either inst level (if you have the original top switch version and select it) or line level and also includes the stereo chorus (which is added after the Fx return and where the MP-1 becomes stereo (well pseudo stereo). If you are not running stereo
, then that may be why you find the level drop slightly with chorus on (and you are missing some of the magic BTW) as half the chorus is going to B (albeit inverted).