I have two 4-ohm 212MBE-II cabs that sound great individually and together powered by my 4/2-ohm Yamaha BBT 500H amp. Found one in a small shop the other GK customer service helped me hunt down by telling me where they had shipped the last few of the discontinued cabs.
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Whoa Daddy! That's a monster rig!
Good sleuthing to find its mate.
I run my Mesa WD-800 through that cab these days, despite it being rated at 800 watts.
@agedhorse has advised that running this amp in 2 ohm mode into a 4 ohm load will produce 600 watts. That does exceed the cab's rating, but I've never needed more than 11:00 on the master and it has never showed any signs of stress. That was to keep up with two guitarists on Mesa combos, keys and loud vocals through PA, and a moderately loud drummer. (I was really glad I brought the earplugs!)
Before the Mesa, I played a 500w Markbass Big Bang head through it, which could get obnoxiously loud. I never got past about 1:00 on the master without it just being unnecessarily loud, but again, the cab never showed signs of stress. It was just shaking-the-house loud turned up just a little over halfway. And in band situations, I was never even able to turn it up
that loud without getting "the look". Your GK head is also conservatively rated, like the Big Bang, so I'm pretty confident you'll only need one. I've played the MB500 and while I haven't played them side by side, the GK amp leaves nothing to be desired from a volume standpoint. I know that if I ever need more than what this cab can give me though, I'll go through FOH. I guess the point is, if you can find one, this cab should serve your needs and pair well with your GK head.
Good luck on your search!