I don't understand your plan to have 3 different cabs. Why not do one bigger cab or a pair (or trio) of smaller ones (ie 2 2x10s, 3 12s, etc...).
And have you tried all the gear with all the gear? You know you can try each head with the other cabs, right? Before selling or buying anything, first try all the various combinations together.
Anyway, in your situation, what I might do is keep the GK head, switch to a 2x12 NEO cab (should be a little lighter and definitely smaller than your 4x10 and handle close to the same power, I think they're rated at 600w), upgrade the Mesa head to the + version, and then swap whichever of the mesa cabs you like least for another of the one you like most (but I would probably go with a pair of 15s just to have something different than the GK).
That shouldn't cost you too much money out of pocket, and you'll have essentially 6 different rig variations you can use as the mood strikes, as well as giving you a full backup should anything go down before a gig (and you can at least take the other head for any on the spot issues).
And have you tried all the gear with all the gear? You know you can try each head with the other cabs, right? Before selling or buying anything, first try all the various combinations together.
Anyway, in your situation, what I might do is keep the GK head, switch to a 2x12 NEO cab (should be a little lighter and definitely smaller than your 4x10 and handle close to the same power, I think they're rated at 600w), upgrade the Mesa head to the + version, and then swap whichever of the mesa cabs you like least for another of the one you like most (but I would probably go with a pair of 15s just to have something different than the GK).
That shouldn't cost you too much money out of pocket, and you'll have essentially 6 different rig variations you can use as the mood strikes, as well as giving you a full backup should anything go down before a gig (and you can at least take the other head for any on the spot issues).