Hey uraniumeilly, Just to confirm, MP-1 > MT200 (bridged mode) > GCS-3 > ? (headphones?, DAW input?,
).
First thing to note from the manual (
Please note that the GCS-3 is NOT A LOAD BOX, and the power amplifier must remain connected to the speakers or have a proper impedance load connected to it in place of the speakers.) So the Thru output needs to be plugged into a cab or a load box (that provides the same load the cab would (minimum 8 ohms for MT200 in bridged mode)) or you risk damage to the output stage of your MT200. If you just used one of the MT200 speaker outs (in stereo mode), you need to have either a load box or a speaker cab connected to the GCS-3 Thru out (so the amp "sees" a load). The amp needs a speaker load (cab or load box) or the output stage will blow up.
The GCS-3 lets you "peel off" a cab emulated signal to feed line level things (e.g. mixer input, DAW input, full range powered speaker etc). So I'm guessing you want the amp sound as well as the preamp sound in your cab emulated signal. This is fine but you must have a load on the amp. For on stage the GCS-3 is designed to go between either the preamp out and your amp (and feed the cab emulated signal to the PA or full range monitor) or between you amp output and a cab to "peel off" that signal for the PA or full range monitor but still feed the cab. Some feedback is normal at high volumes just like when you crank up a guitar amp, and they emphasize that in the manual. I suspect that this is because some cab emulators don't have any/much feedback like you normally get on stage when it's cranked.