Magnepans with Bryston 7BSST

I've now got a Bryston 4B running my Magnepan 3.6's and a 3B for my Mag CCC3 My 1.6 rears are sitting idle as I sold my rear amp. What i'm wondering is how much overall improvment will I get by swithching out the 4B for a pair of 7BSST's? I'd then move the 4B to power the rears. This is my plan and I'm ready to make the purchase. But now I'm wondering if I should maybe get a new 9BSST to run them all. What are your thoughs? Space is not a problem and I actually like the look and feel of all seperates.