Hey there Jax,
I think power wise the 4b is more than enough for your room and speakers but somthing to keep in mind is the power supply difference. The 7s and the 14s offer better regulation and isolation than the 4b so you will get a quieter background and more in the way of dynamics and ultimate clarity. The alternative and cheaper route would be the 4b with a bit power isolator. As well, given the room you have, I would take two 7s over a 14 any day. There is only a marginal difference in price and the 7s are considerably stronger when it come to the toughest loads.
What is the NAD not giving you that is bringing you back to Bryston. You had a 4b sst2 didn't you??