Well you are correct, it is not a load, you still need connect the speaker to it. I guess this works great for a W/D/W setup.
Sorry El Chiguete, but what's W/D/W ? please pardon my ignorance
Ok I think I get what you are wanting to do, you want to add the "mono" warmth of your Epiphone Valve Jr (a guitar amp (i.e. preamp and power amp combined in the one box, as all guitar amp are (tube or SS or hybrid, except the vJnr power amp is class A and most guitar power amps are Class B (push pull)))) but have it stereo (chorus, effects etc) in the end ??
So, to do this in the MP1 loop, MP1 send > Epiphone Valve Jr in, Epiphone Valve Jr speaker out > GCS-3 in (-30db pad), GCS-3 out (cab sim off/out) > MP1 return, MP1 A/B out > effect L/R in, effect L/R out to stereo power amp > 2 cabs (this will work as a live set up, or GCS-3 cab sim on/in and effect L/R out > desk or recording inputs for recording).
While above should work, the gainstructure in the loop (any loop) is all wrong and the MP1's loop isn't particularly accommodating as it's 1 dual ganged pot (turning it up increases send and decreases return levels and visa versa), Unfortunately the Epiphone Valve Jr doesn't give you many options from what I can see in google pics (the schematic looked promising (preamp out/power amp out) but you can't get in and out of it except guitar in/speaker out ?? pls correct me if I'm wrong or you have a more souped up model??), so you'd want the MP1 send to be inst level (milli volts) and the MP1 return to be line level (volts), isn't gonna work well with the MP1 loop so major gain miss-match (if MP1 had separate send/return level controls, then different story). This is part of the reason I use a small mixer to plug it all together, you have individual control of all the inputs and outputs so can gain match as required...
Anyway, to get the Valve Jnr into you sound with the MP1, I think there are there are better ways to achieve this (another conversation....).
Also, unless your stereo effect unit has a true analogue pass through for the MP1 A/B outs (direct signal) it's not optimal to put it (chain it) between MP1 and power amp/record ins. This is because most digital effect devices A/D the signal on input and then D/A them at the output (some/few have analogue pass through). I know I rant on about this (see other posts) but my mate MikeB has just vindicated my opinion on this by using an ART splitter mixer (also see other posts) so now his TC Gmaj is "mixed into" the analogue MP1 output signal (he used to chain through it, as do lots of MP1 users, which, while convenient, and seems logical (is it called the 4 cable method ??)) is a bad idea sonically (depends how much you care though I suppose). He was quite skeptical at first, but now he's done it, he said there is a significant improvement keeping the MP1 analogue all the way to power amp (as I suggested to him there would/should be). Anyway he will no doubt post his findings/experience at some point.
So he's used the ART splitter and 4 x stereo to 2 x mono jack (insert) leads to achieve this (all the ART jacks are stereo), works fine for one stereo effect unit (as I suggested in other posts). Big improvement in sound, but unfortunately he loses the ability to control his master vol via the TC with midi CC foot controller. And to combine more than one effect unit (lets call your Jnr idea a (louder, mono) warmth effect unit) so it was also sent to the other effects, you need a mixer (8 channel, 3 or better 4 effects (Aux) sends)