I'm gonna have to disagree with most of the comments & say your guitar is the place to start.
If you like the SG & how it plays, get it modded for sure, it will get you way closer way faster than any speaker will IMO.
It doesn't matter if you have a 65 twin that's been modded to be a preamp>mc2300, it's not gonna sound like 79 Garcia with an SG that has 2 humbuckers, it's just not.
I have a les Paul Double cutaway & and epi hollow body. I bought a 67 twin reverb & usin those guitars it sounded like 69-71 Garcia. Had it preamped & ran it through a crest power amp into JBL's & it still sounded like early Garcia.
Then I had the LP modded Jerry style. Put an sds-1 in the middle(due to room available) and super 2's splittable in the bridge & neck positions, the 3-way pup selector changed to a 5-way rotary selector & the volume knob as a push-pull to split the pups to single coils or humbuckers. Also had the Buffer installed.
The second I plugged that guitar into my amp I was like ahhhhhh, THAT'S what I've been looking for in tone. It literally brought it from a guitar that I could not get to sound the way I wanted through any amp to a guitar that sounded exactly how I wanted it through any amp, it was night and day.
Having that middle pup is CRUCIAL for getting the Garcia tone, IMO it's just not happening with any 2 humbucker guitar. The only double 'bucker guitar i've ever played that has a similar tone is a PRS that also has a piezo pup with a blend knob, but that's thousands of dollars and still not THE tone you're looking for, more like '91 Garcia.
At this point I hate playing any guitar that just has two humbuckers, it's just so restricted tonally. That's the beauty and genius behing Garcia's setup, it truly is the best of both worlds. Those certain songs that you play where you like playying on bridge humbucker you still have that option, it's like a true hybrid between a strat and a Les Paul, don't get any better than that!
You''ll get the most bang for your buck by having that SG modded for a third pup in the middle, you''ll be VERY happy thatt you did. This is assuming you have the neccesary playing chops but you'll be able to get a pretty damn convincing Garcia tone even out of that Peavey, but run it into your Twin and you'll be in tone heaven, I swear.
Personally I would go with the SDS-1 in the middle rather than a single coil super 2. That's what I hae and I couldn't be happier with the tone. The difference between the super 2 split and the SDS-1 is the SDS-1 is hotter, which is what you''ll want on that SG. You gotta remember that Tiger weighed what 13lb's? THat's a lot of wood there which provides tone, defintetly on my LP DC, which is like 7-8lb's that hotter signal makes up for that loss of weight, at least somewhat.
The bottom line is no matter what amp you have you're never going to get a 79-82 tone with an SG that has 2 humbuckers. You MIGHT be able to get kinda close, but you're never gonna get to where you really want it until you get that third pup in the middle and you make the HB's splittable.
IMO the guitar is the key element to the tone, the JBL speaker is just the icing on top, that's for when you're going all out. Your guitar is still going to sound like an SG when it's going through JBL's, it's certainly not gong to sound likke 79 Garcia.
First thing is get the guitar tiger-ized,, you will NOT regret it. There's nothing else you can do for a few hundred bucks, or even thousands that's going to get you closer. That SG will sound money even through the Peavey once you have 3 splittable Dimarrzio's and a 5-way selector. That is CRITICAL for the Garcia tone.
Next step, since you already have a twin, is getting a mcintosh power amp. There's Just no amps I've ever heard that sound as good as a mac, they're just awesome going through almost any speaker.
I think the last step is getting the JBL, thats the icing on the cake after you have the guitar tiger-ized running into a nice preamp into your mac, that's when you wwant the TRULY authentic Garcia tone.
But remember, an SG with hb's is going to sound lke an SG with hb's no mattter what amp or speeaker combo you choose, it's never going to sound like Garcia in 79.
Get that bitch modded ASAP with the 3 dimarzio's, buffer and splittabl hb's,, you'll be so glad you did and so much closerr to the tone you want than any amp or speakers will get you, as evidenced by the fact that you already own a twin.
All JMHO and YMMV, but trust me, tiger-izing your SG is definitely gonna be the most bang for your buck.