The stock speaker is a bit dark for humbuckers, I find that the number 8 Plexi setting around 5-7 gain 10 bass and 7 treble works well for an ACDC crunch but that is with single coil pups. With darker pups like yours, you might want to try something a little higher pitched like the number 7 Vox with the gain around 5 for some crunch.
I have the Jensen CR8 speaker which is too bright for single coils, but I bet would be simply awesome with your Les Paul. Don't give up on it yet, I have found that this little guy can change sound and dynamics with just a tiny change to the gain. Try setting your gain on 1 with any channel and work the gain up in half turns till you find the sweet spot, it's out there =)
Edit** Rae, When I bought my VCXD, I ran it through its paces with a floor model Epiphone Les Paul, and its a whole different animal, mucho sustain and it's darker, but sounds equally good as it does with a single coil guitar. Different sounding for sure, but really nice! Loved the Vox sound with the Epi-Paul, it just sang man, and had that nice thumpy bottom end you would expect with a Les Paul.