I just picked up my Marshall JCM2000 TSL100 half stack and Im trying to get it up and running with my Boss GT6 via 4 cable method. Here is my connections/setting so far:

Cab to head connections
1. 16 OHM Only output on head > 16 OHM input on cabinet

Cab/head settings
1.Cabinet switch set to Mono
2. Head switch set to 8 OHM
3. Loop level button on head is pressed in for loop A

Boss GT6 to head connections
1. Guitar output to Boss GT6 input
2. GT6 L (MONO) output to head effects loop A return
3. GT6 External Send to head input
4. GT6 External Return to head effects loop A send

I have the GT6 setup with a blank patch basically. No preamp on, no effects on whatsoever. The only thing assigned is the pedal as volume. Master volume on back of GT6 is around 50%, patch level is around 50%. Head volume is at 1 for the clean channel...messing with ALL volumes does very little. Turning everything up all the way, and turning the clean channel volume to 5 or 6 its still too quiet.

If I use a patch with preamp settings and whatnot programmed, it works fine, but I want to use my GT6 simply as an effects pedal, not an amp modeling pedal like it was designed.

What am I doing wrong? I know everything is hooked up right, its gotta be settings on the GT6 itself. What do I gotta do to get some real volume out of my setup WITHOUT using the GT6 preamp/speaker section?
Thought I might add, I tried using the GT6 in front of the preamp like normal (guitar to GT6 input, L output to amp input) and it works fine with no preamp on the GT6 or anything else selected. I can turn the effects on the GT6 on individually, and it works. I can turn on amp modeling or distortion modeling, and it works like a charm. Its definitely something to do with my connections.

Im a little confused as to the point of the 4CM now. It seems as though just hooking the pedal up "regularly" accomplishes everything I need it to. So what gives? Why wont it work in the 4CM?
The effects loop for the GT6 needs to be ON.

I believe there is a mix knob for the TSL100, since it's a parallel loop.

4CM allows you to use time based effects like reverb and delay, as well as modulation effects AFTER the preamp of your amp. This way you can use the TSL's drive and dirt with effects. That's the point of 4CM.
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Anyone have any idea how to turn the gt6 effects loop on? Ive searched high and low and cant find it
Just press the button twice.
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Thanks for the help...i have a few things im gonna try tonight. Before i didnt have the OD on the pedal set to external and i dont recall if i had the FX button on the Marshall foot switch selected or not but i dont believe i did. I assume one if not both of these would cause my issue. Can someone tell me why it would cause this issue without OD being set to external? I figured as long as youre not using the GT6 distortion it wouldnt interfere (i had it switched off completely)