Good to hear you are running stereo, and 4 x split stacks
, should sound awesome
.
So there are 2 main sorts of Fx "loops":
Serial (as in MP1) where the Fx unit is inserted into the signal path, with MP1 there is always signal on the loop send, this signal is after the tubes/gain stages and eq, the loop return deeds into the stereo chorus and is then split into A and B (IIRC B is the A signal inverted so creates "pseudo" stereo), so the MP1 loop is a mono loop and predominantly what the pitchtraq was designed for (as it's a mono device). When the loop is on, the direct signal goes through the loop only and you need to adjust the desired wet/dry mix in the Fx unit(s), you can chain other mono Fx units in the loop (e.g. delay) being careful to get their individual input/output signals the correct level and also their wet/dry mix settings. This is fiddly but it does work. Also the pitchtraq lets you use a delay plugged into it for various cascading/arpeggio Fxs (another option with what you have.
Parallel loops work a bit differently, the direct signal isn't interrupted by turning on the loop, rather it is split so it also goes to the loop send (well it usually already goes to the send, turning the MP2 loop on turns on the A & B loop returns). The loop return then feeds into an internal mixer stage that blends the amount of return signal with the original direct signal. So with these kind of loops you adjust the Fx wet/dry mix to be 100% wet (Fx), no dry and then balance the amount of Fx you want with the return volume setting. The MP2 has a stereo parallel Fx loop, again there is always signal at the sends (A & B) and when turned on, the return signals A & B are mixed back with the dry signal within the MP2, so you adjust how much Fx you desire with the A & B loop return volumes (set for each patch). With MP2 the A and B loop returns are on the separate MP2 channels (A & B) as at this point in the MP2 the signal is already stereo. So if you put a mono Fx unit on say loop A, it will only effect channel A, channel B will be just dry signal (so with pitchraq in this scenario ch A would be pitch shifted, B would not).
Using a small stereo mixer with a couple (4 is good) of Fx (aux) sends lets you control it all in the mixer and also compensate for stereo or mono Fx units. So you plug the preamp MP1 or MP2 A & B outs into say mixer channel 1 & 2, use the channel Fx(aux) sends to feed the Fx unit, plug the Fx unit output back into the mixer (say channel 3 if mono or 3 & 4 if stereo), adjust Fx unit to 100% wet, and mix the dry and Fx signals with the channel volumes (faders) and plug the mixer main outputs (L/R) into the B200s A & B inputs. Much like you'd set up a Band PA mix and set you L/R pan controls accordingly.
So how I used to run my rig (I've never used the MP1 or MP2 Fx loops) (initially with MP1 then later MP2):
I use a Yamaha AM802 desk, 8 inputs, 3 mono Fx(aux) sends (1, 2 & 3), stereo out.
MP1 or later, MP2 A/B outs > channel 1 & 2, ch 1 Fx send to Quadverb L input, ch 2 Fx send to Quadverb R input, ch 1 Fx send 3 & ch2 Fx send 3 to IPS33 smart shift input (as is mono in). Quadverb L/R out into channels 3 & 4, IPS33 A & B outs into channels 5 & 6. Main mixer outs to B200s A/B inputs, B200s outs > 2 x split stacks.
MP1 or MP2 > channel 1 & 2