Re: 50 watts or 100 watts?
Ok, a 238992839 page thread on this simple question... wow!
I'll try to sum up the differences between the 50 & 100W and different speaker cabs:
Headroom: 100's power section will stay clean longer. Meaning you'll have well defined power section vs an overdriven one. This CAN matter... especially if you wanna play clean cleans or if your sound comes mostly from the preamp or pedals.
Volume: 100W@9 will roughly equal 50W@10, so this won't be your problem at all.
Speakers: Depending on what you'll use, the 100W might drive your speakers more resulting in them opening up more. But the other way round (this won't be the matter with your 4x12) they can kill small cabs like a 2x12 loaded with low power speakers.
Regarding speaker cabs:
Some people think a 4x12 is way louder than a 2x12, which it is, in fact not. The volume difference is the same as with the 50w vs 100W... about 10% every tome you double the speakers... what WILL matter however is, that the larger cab "covers" a larger area, therefore you are more likely to stand "in the field" of the cab. Especially, if it's standing on the ground.
Another thing is bass response... since the 4x12 is larger, the bass will be bigger, but slightly less defined sounding.
Speaker sensitivity ratings are also very important since 3dB means about double the power (not volume once again)... so 4 97dB speakers are roughly as loud as a 100dB pair.
Yeah, so these are some factors to consider.
Can't you run a 2x12 JVM combo
with a 4x12 1960A...or B ?
A
6x12, sounds good to me

. . . . very GOOD actually !
James
If he's gonna run it trough a 4x12 anyway, I wouldn't care about the hassle of possible higher microphonics, more tube heat and (not sure about that) the open back pair of speakers. Also he'll have to watch out not getting an OHM mismatch here since combos are usually quite limited with that.
Best thing really seems to be trying out a bunch and/or looking what players at the venues you'll be playing usually have to get an idea.