Hey Micky, VERY Cool unit
so from what I can see/read at that link I would try patching it MP1 A/B outs > Art Splitter Input, Art 1 out to Midverb ins (L/R), Art 2 out to LXP ins (L/R) then Midiverb outs (L/R) to Art input 3 and LXP outs (L/R) to Art input 4, Art Mixer out to power amp (L/R) inputs. Adjust in/out gain and wet/dry mix accordingly on Midiverb and LXP.
This setup would require a bunch of 1/4" stereo (Tip Ring Sleeve (TRS)) to 2 x 1/4" mono (Tip Sleeve (TS)) jacks (typically used as insert leads in studio or PA applications so you can buy them or make them (TRS tip to mono jack1 tip, TRS ring to mono jack 2 tip, TRS sleeve to both mono jack sleeves)). This patching needs 6 insert leads and takes advantage of the Arts all stereo inputs and outputs. The stereo jacks plug into the various Art inputs and outputs and their respective mono jacks into the inputs or outputs of the 4 devices (MP1, Midiverb, LXP, power amp). This setup should give you everything in stereo. You can probably use Art inputs 1 and 2 for the stereo returns from Midiverb and LXP (not sure would have to experiment or see the schematic, although, thinking about it it would work). An alternate patching might be: 2 x insert leads (MP1 A/B > Art in, Art out > power amp ins L/R) and use standard mono patch cables between Art <> Midiverb and Art <> LXP (this would use all 4 Art ins/outs as well as the Art main in/out). With both above patchings you have to do the wet/dry mix in both the Midiverb and LXP.
Or another (probably (definitely) the best) alternative: MP1 to Art splitter in and Art 1 out to Art 1 mixer in (stereo cable TRS <> TRS (maybe you don't need to cross connect this ? if the dry input signal (Art splitter in) is internally patched to the Art outs that are not used/plugged into then you don't)) this is then MP1 dry signal, Art 2 out to Midiverb ins (TRS <> 2 x TS), Midiverb outs to Art 2 mixer in (2 x TS <> TRS), Art 3 out to LXP ins, LXP outs to Art3 mixer in and Art mixer out to power amp ins (TRS <> 2 x TS). So Art 1 vol is MP1 dry, Art 2 vol is Midiverb and Art 3 vol is LXP and both Midiverb and LXP can (should) be set to full wet and Art vol 4 turned off (as it will also be MP1 dry if I'm right about the internal patching).
How much do the Art splitmix4s cost ? You can run up to 3 stereo effects (or mono in stereo out gadgets or mono gadgets) this way and keep the analogue signal pure and all in stereo. Very versatile unit.