mesa subway speakers

I recently got a Mesa TT-800 amp head and have 3 cabs. A Mesa Subway 2x10, a fEarful 15/6, and a Ampeg SVT410HLF. All sound great, and staking the 2x10 on top of the 4x10 is really amazing. My conundrum is, that I feel I should trade the Ampeg 4x10 for another Subway cab. So if I stack another...