I run MB2's in bi amp mode with a pair of 7B SST per speaker.
More information in the image than with 4b's
I also have BB5 running tri amp 4B,4B,7B
Oh and as of today I have BB5 with XBD fully active, thats being powered by 4,4,7,7 per side
Active speaker systems is really what my work has always been - (Pro audio)
So whats best for you...........
I would keep the 4B and let it run mid/hf in bi amp, one side per ch and then add a pair of 7B one for each bass, a 14 is also possible but the 7s allow you to place them close to the speakers,
Active - OK, for me this is an easy route, its what I have spent most of my life as a Pro Audio engineer working on, BUT I would say that thought should be given to adding the XBD in MB2 prior to thinking about active.
I would not stick a 3B on the mids, I used to run a 3B on the HF but thats now a 4B on the BB5 system. Home alone and enjoying things the 3B used to clip (Sorry for that rest of village!) 4 on the mids is amazing, 7 on the mids is just another world.
The active PMC / Bryston system is NOT off the shelf buy it, plug it in sit back and relax, done correctly ALL the bryston equipment is modified by PMC, The PMC kit is specific for the active system as well - So it is possible to go from MB2/BB5 towards an active system but best option is to start with the active system - AGAIN my money would be on the MB2 XBD option rather than MB2 active.
BB5 XBD active being the ultimate system
ALL of the above assumes a 2ch system.
I am in London, UK.