Although the Beethoven Concert Grand has greater power handling, more drivers and can ultimately move more air and play louder than the Mozart Grand, it has 1dB greater sensitivity. If there are not a lot of hours on them, perhaps the driver suspensions and voice coil spiders are still a bit stiff (and perhaps the new woofers are stiffer than the Mozart Grand's midbass drivers were when new) and the volume hits a point where the resistance is not a factor.
The 9BSST may run out of steam driving the new speakers to higher levels than you could previously, but I wouldn't expect a lack of power to have anything to do with the volume transition you're experiencing, so hopefully it is just a matter of some additional mechanical break-in hours.
Steve