Yep, and you were right from the beginning that the MX-A amp was akin to the RX-Z11, i.e. an AVR, albeit a very powerful AVR. So not surprisingly, when bridged it could not get close to 4X or even 2.5X the rated output into 8 ohm. No AVRs can, so no AVR derived amps can either.
Was it because it was tough for you to pick it up safely by yourself?
Seriusly, the brochure and manual says 132 lbs 4 oz, could that be a typo? If not, that fact alone tell us it isn't an AVR derived power amp for sure.
If true, 132 lb for a 10 channel power amp is a bit much for most home HT.
For about $1K more, an AT528NC (200WX8, $4.6K) for the surrounds and AT543NC (500WX3, $3.9K) for the LCR would be a more practical solution, but I guess you are not quite ready yet to join the class D club.
By the way, the little 100W Benchmark ABH2 is one that claims "The bridged MONO mode will deliver almost 4 times as much power into an 8-Ohm speaker, and will do so without any increase in distortion." That sounds incredibly good, may be you should verify that on your bench.