I'd get 2 x G12M greenbacks and 2 x Celestion V30s, they combine well together.
not really
they pair well in different cabs/setups
the v30's are way louder and more midrange dominant than the g12m's
they overpower most speakers in the market, unless you pair them with different voiced speakers, like the g12k100 or the g12t75
not that a cab with v30's and greenbacks together sound bad, but you're wasting 70% of the greenback tone, since you won't hear them
a cab with greenbacks and g12t75's would be great, as their voicings fit almost like opposite sex genitals on each other and they have the same sensitivity and magnet weight
first you gotta figure out what kind of speaker you like the most, and then what kind of speakers sound best with your amp
I love the smooth and "small" range of the greenbacks
they are all about crunchy mids, but not honky or aggressive
the vintage 30 has a punchier low end, in your face mids and a bit harsher top
I love them as well, mostly for recording and for some live amps, but others can sound way too honky
for smaller cabs, I love the mesa celestion black shadow (custom variation of the classic lead 80), and it sounds amazing with hot rodded marshalls
lots of low end, strong but smooth top, flat non-scooped mids
higher wattage speakers with flatter or scooped midrange tend to sound better for cleans
the v30 is hard to beat for distorted rhythms and the greenback for fat lead tones
funnily enough, my modded vintage JMP sounds better in person through a stock laney cab (not sure which speakers) than through an expensive mesa 4x12 with v30's (the mesa always wins for recording, though)