I downloaded the manual for the Bryston A3 at the link below. The impedance and listening window curves are slightly different between the standard M60 and the A3. The standard M60 alpears to be a tad mote linear between 100Hz and 200Hz. Low frequency performance doesn't appear to be any different but I'd bet at higher SPLs the HP woofers in the A3 perform better.

https://bryston.com/passive/a3/

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated