I can only string together what I was able to figure from the forums and talking to the guys at Mesa Hollywood.
Appears that the Mini Rectos were optimized for the V-30's in sealed back cabinets, and when they decided to do a combo the C-90 wasn't to their liking in the smaller cabinet, and the V-30's are really not well suited to open-back applications (at least not the Mesa version, which I personally think is the definitive V-30).
Sidebar: I have a 19" open back with the C-90 in it as well as a 1x12 Recto cab with the Mesa V-30 and I think they both sound fantastic with the Mini Rec (and my Mark 5:25 as well). Main difference I can hear is the Recto cab has more punch but is more focused/directional while the C-90 open back sounds bigger (huge in fact) but the low end is not quite as tight. Neither has "more" low end. The V-30 is definitely better with high gain, but the C-90 is much better with cleaner tones and I would say is more versatile.
Evidently they had been working on a homebrew speaker for quite some time and went through a zillion prototypes and so when they did the Rectoverb combos they decided to fine-tune it for that application, where they would have the best attributes of the two Celestions in one speaker. They are made by Eminence, and the combo that I played through (at pretty low volume TBH) definitely sounded great, seemed to have plenty of punch and managed to sound larger than the cab would suggest and not boxy. In reality, they are probably saving a lot of money using these speakers over the Celestion models too.
I am more of a head/cabinet user these days and I like to mix and match. If I could go with only one cab it would be the sealed back with V-30(s) for this amp (I'd probably choose the open back for the Mark 5). Another thing: I also play mine through a 4x12 with Eminence man-o-wars and it is completely giggable with that cab, no volume problems at all.
