Reference 3A De Capo BE ?
On the other end of the spectrum maybe ATC SCM19 v2.
The new amp may not have quite enough power power to drive the M5's. If you don't have a big room and listen at high volumes, you'll probably be OK. If you do a lot of off axis listening, you may want to keep the M5's. If not, the Vandersteen Model 1's would be a good choice. Easier to drive, and they do things the Mirage can't do. But they have a sweet spot, and if you're not in it the difference will be noticeable.