Hey, money is good!
Yeah it's always a risk buying a used amp over the internet, but really Guitar Center's used policy makes it about as safe as it can be. You have 30 days to blow it up. If there's something wrong with it, you'll know. You'll get a full refund. The JVM line has been around since 2007, and it's had a very good track record so far. There are only a couple things to look out for. The very first batch of JVMs in 2007 were iffy due to a redundant fuse holder shorting out, but they fixed that at the factory within the first month of production. So unless you get a JVM built literally within the first few weeks of production, you will be good to go. Those fuse holders don't even exist after that very first run with very early 2007 serial numbers.
The other thing, and this isn't a deal-breaker or anything, is that the later models have sturdier pots. The pots are PCB mounted, but the board is mounted to the chassis. It's very sturdy. The earlier version have PCB mounted pots as well, but the pots just poke through the chassis instead of being mounted directly to it so the pots are kind of wiggly and feel a little flimsy. Mine has the chassis mounted board and the pots are sturdy and smooth. My buddy has a later 2007 and his pots wiggle around a little. I also don't know if his has the fuse holders or not, but he's had no problems.
So all in all, it's a reliable amp. Mine is a 2010 and I've had not one single issue with mine. Not anything.
It's hard to say what speaker will go best with the JVM because it's capable of a huge variety of sounds. Like pretty much all modern Marshalls, I think it's generally designed to be paired with the standard G12T-75. The power section is very tight though, very tight, so it's not an amp that needs to be "cranked". Louder is absolutely better, but it's not like an old Marshall that relies on pure brute loudness. The JVM is probably one of the better 100w amps out there that will actually do decent bedroom volumes with just the volume knob and no other amp trickery. So it is quite good for quiet practice, or full on loud recording and gigs. When I'm dinking around at home quietly, it's with the JVM. My Plexi, 800, and JMP 2204 are almost completely useless at volumes that don't knock walls down. The JVM seemingly gets most of it's sound from the preamp section. The power section does influence the sound, but it has to be very loud to get there and the payoff isn't quite the life-changing experience that a 2203/2204 gives when it goes into massive meltdown as the volume goes up. I suspect the new DSL line is the same way. I've modded mine to have adjustable negative feedback reduction which lets the power section break down sooner. The resonance and presence controls sort of do the same thing, but the mod really gets it done.
Anyway, you already have all the speakers that will work great with the JVM.....you just need more of them. Like I said, the JVM doesn't have to be cranked, but you don't get an amp like that to play at low volume. You wanna move some air. The JVM could make short work of destroying your Greenback in a 1x12. That's my personal favorite speaker, but you know, it's only 25 watts by itself. What makes them sound great is also what kills them. You're gonna want a 4x12 if you want to use Greenbacks. I worry about my Greenbacks a little every time I flick on the Plexi. But hot damn I think they sound good. The Vintage 30 sounds really great with the JVM's higher gain settings, and it would probably survive in your iso cab unless you go crazy with volume. Your G12T-75 will also do well on it's own with some volume restraint, although I think the Greenback and V30 sound better. Just stay away from Celestion's H/K-100. Those 100w speakers might seem like a good idea, but they are ass. And don't forget WGS. They make nice affordable Celestion knockoffs.
