The Bryston is a great amp, but one thing you won't ever know for sure is how much of the difference you heard was from age of the Adcom.
Whereas a CD player usually either works or doesn't, an amp is (at least partly) a device that gets "consumed" with use. If your Adcom had been driving the Maggie's for a decade plus, there is a good chance it was not performing to original specifications. Replacing the capacitors is a typical maintenance item for such a scenario (and something you might want to try if you still have the Adcom or passed it on to a friend).
Hopefully the Bryston has enough extra "oomph" to hold up well under the load of the Maggie's.
@PENG , am I presenting a legitimate view of amps and their deterioration? Thanks!