It seems we went way off topic here.
but at least the original question was satisfied early.
In general terms of course. Do not put an MC90 in a sealed cabinet! It will sound terrible. You would not want to hear that unless you like flub followed by the crackling tone after you blew out the voice coil (I believe this is what did in the MC90
) I have experimented with this when I got an extension cab for my Mark IV (after removing the MC90 of course which was the donor speaker for the extension cab).
Most people do not have EV's in their cabinets. Hands down they are one of the best choices to get the boogie back into your amp. The Mark IV though a 412 with EV's is out of this world. It actually sounds great though a traditional sized cab with V30 (that surprised me). However with that amp, I settled on a Fane AXA Studio 12L which is similar to the EV classic. I bought the Fane after completing the 412 with EV and wanted to try something different at a lower cost point. Turns out neither the EV or the Fane really fit the Mark IV unless it was front mounted. Had to modify the grill frame to accomplish the task. Was quite easy with a router. I probably could have gotten away with rear mounting the EV with the Mark V, but considering the Power tubes were at the same position as the Mark IV, as well as the preamp tube shields, I wanted more room available to change tubes without having to remove the chassis. Turns out the preamp tubes on the V sit farther back than they do on the Mark IV.
The Mark V on the other hand, I feel does not sound very good though the V30. That is definitely the Nasal tone. It gets worse with a V30 in a combo. I had experimented with the V30's before I bought the RA100 which is the only Mesa amp that V30's make any sense. I even tried the Roadster through the V30 cab, seemed a bit muffled. However with the Mark V, Mark IV or the Roadster running both the EV and V30 cabinets as a full stack sounds great. I tried mixing EV and V30 in the same cabinet but that did not perform well. Separate cabinets was a better option. The RA100 is the only reason to keep the V30's as it does not sound all that great through the EV cab.
However, at low to moderate power levels the MC90 is not bad at all. I basically suffered with this speaker for 14 years in the Mark IV, 8 of those years I had an injury to my left arm and thought I would never play again, so there was no rush to modify the amp if that were the case. I did not take the opportunity to hear the Mark V with the MC90 when I did the head to combo conversion since I already had a two EVM12L black label speakers to use. From clean cleans to swamped FAT tones, including the Tweed (also having the right tubes in the amp) the Mark V shines and performs well. Push the envelope in CH2 or CH3, the EV will transform the anemic Mark V combo with MC90 into a new amp capable of delivering the goods for gigging and almost concert (a 1x12 just will not do for that). Take that one step further and add an oversized and ported extension cabinet and you will have the same capacity of a sealed 412 and yet retain the qualities of open back tone. My Mark V rig consists of just that, Combo and extension cab and WOW does it compete with the EV stuffed 412 sitting right next to it with a Roadster head sitting on top.