Depending on the disc, the center surrounds may not get a lot of load -- but some discs (and probably more to come in the next couple years) may start putting as much data into the center surround and the surround currently. If you already have a 750, I think the choice is pretty easy to simply track down a 2-channel amp for the center surround (I got an old 100WPC Parasound HCA-800 off Audiogon for about $125). The B&K at 60WPC may be a little anemic when paired with the 165WPC 750, but then again it may hold its own for most material (for comparison, the standard 1050+750 combo uses the 1050 to drive a single center surround at around 90W).

If you are starting from scratch, the choice between 750 and 770 becomes cloudier. You might consider getting the 750 to save money and evaluating the B&K's performance, then if it doesn't get the job done replace it with a more robust 2-channel (which can probably still be done for less than the cost of the 770). On the other hand, the 770 may prove a little more future-proof, as additional channels keep getting fitted into the home theater realm and existing channels used more and more aggressively; three years from now you may need seven channels of full range, heavily-used amplification.