Wow, I've never heard any sort of compression or phase cancellation unique to the Epifani stuff.
However, what you guys might be talking about is comb filtering, which is typical of a parallel speaker config, but not a vertical one, but that should come and go as you walk a circumference around the cab, and it shouldn't be a huge problem. I could see that not happening with the 212 since they aren't next to each other (and since you won't be going up and down an elevator on stage any time soon likely).
The 310 is not exactly a parallel config, but still has drivers somewhat adjacent, so I suppose it could happen there too, but to be honest, I've never noticed it myself.