I've not used/heard the GSP1101 but I just had a read of their sales blurb. While on the surface the way you've patched your rig makes sense so you can have stereo effects (from GSP1101). However, using the MP1 in the GSP1101 loop IMHO wont work as well as having the MP1 as your main preamp and the GSP1101 (or other multi effects device) in the MP1 loop (but effects not stereo
), this will however, open up your MP1 to it's potential. Part of the problem your experiencing with the MP1 sounds is IMHO the GSP1101 (try leaving it out altogether, ok no effects except MP1 chorus mmmm but I recon it will sound heaps better, particularly live). Seems the general trend with other MP1 members is to chain a decent stereo effect device (Quadverb, TC, etc) after the MP1 (e.g. Guitar > MP1 outs L/R > effect ins L/R, effect outs L/R > poweramp in /L/R). While this is a decent compromise IMHO it's not optimal as this way the MP1 is limited by the effect device's headroom and, unless the effect has an analogue pass through for dry signals (rarer these days), the MP1 analogue signal will be digitised so it's at the mercy of the effects A/D D/A converters.
Ok, I just checked out the GSP1101 block diagram in the manual, its loop is after the initial input and its loop returns are before all the effects so here are a couple of ideas based on using the GSP1101 as your effect device (but better/easier to use a dedicated multi effect unit):
1) GT > MP1, MP1 loop send > GSP1101 loop rtn (L), GSP1101 line out (L) > MP1 loop rtn, MP1 outs A/B > amp in (this should give you mono effects but the signal has been A/D D/A converted and (MP1 loop is series) wet dry mix of effects/signal happens digitally, (same for any digital effect without analogue dry pass through)).
2) GT > MP1 > MP1 out A/B > GSP1101 loop rtns L/R > GSP1101 line outs L/R > amp ins (this should give you stereo effects but again all wet/dry mix happens digitally in the GSP1101).
With either of these patchings, the best result will be only adding effects with the GSP1101 and turning off all (or as much as you can) the cloning/modelling stuff (e.g. take 1/4" target output system input from volume post cutting out the stereo cab, 7 band global eq (there's still the parametric eq if you need it) and low pass filter).
You can find the owners manual here (http://www.digitech.com/en/products/gsp1101), it's to large to attach, block diagram on page 55 may help understand my rant
I've posted some other patching ideas for the MP1 if you look around here, one that seems promising is using an Art splitter mixer and 4 insert (TRS > 2 x TS) cables to maintain stereo all the way (some of that rave is in this topic (http://adadepot.com/index.php?topic=217.msg1279#msg1279)), better IMHO but a bit more complicated (again use the GSP loop ins and line outs).