I went from the 4CM to the entire M13 in the loop, with a Boosta Grande and a Sonic Stomp in the M13's loop, and set the loop to "post all". I liked how the 4CM worked and the ability to run what you want in front and whatever else in the loop. But for me, it seemed to color the amp a bit more than I wanted it to. Might have been just me, but I just didn't like how it sounded. Plus, I basically only use it for the mods/delays/reverbs, so no real loss. The amp is a screamer, and I basically use its gain stages as my tone, boosting the front end when I need with a BB Preamp.
My gripes with how everything is working now is:
1) there is a latency problem between switching scenes on the M13 and the amp channel...amp doesn't change right away when I change scenes from clean to dirty, or from crunch to lead. Doing scene changes within the same channel, no problem.
2) I have a comp setup in effects position one of the M13 on all of my clean scenes to remedy the volume issue between the green and blue channels. It has helped to bring the clean (green) channel up to volume with the crunch (blue) channel, but now I have a pretty nasty volume spike during the channel switch. once the channel sets in, it's all good, but it is right at the click of the footswitch on the M13 that I get this peak of volume that irritates me to no end. When I take the comp out, or any effect to boost the clean channel up to volume with the other two, no spike, but now the clean is hardly even present compared to the other two channels. And I had this issue with both cable methods, the 4CM and my current signal path.
I just recently purchased a Behringer FCB1010, so hopefully this will remedy the whole situation. Waiting on shipment still, but until then, I make due with it. Luckily I haven't been gigging for a little bit. So I got time to R&D everything before I open her up on stage.
I hope you do not experience what I am, but then again, the shared EQ on the clean/crunch channels of the 5150 is the reason for this, so I am sure your SLO100 will not have that problem. The latency issues, not sure. Seems I may be unique, but then again, my unit is over 4 years old and I have been gigging this thing regularly for about that long. If I have to, I would purchase another one, for sure. I love my M13!!!
Good luck to you and your tone. For what you are planning, the 4CM is the way to go. And you should have great success with it. It is a fantastic unit, just wish they (Line6) continued with the updates for it. But business is business, and I can see why they stopped! Nobody would buy any new gear, just keep updating the board!!!
Hope this helped! Aloha!