What Im suggesting is instead of 1x 7 channel amp that can do 180w per channel use 4x 2 channel PA amp (like the crown XLI 1500) and wire each speaker direct to an amp channel, no selector or bridging needed.
The XLI 1500 is 330w into 8ohm (soundstage 15 is 8ohm IIRC) which although slightly lower than I would say is ideal it is very much a better usable range. If you can get a similar amp that can do 400w+ into 8ohm that would be better but only if you can find one at the same price. If a higher power amp is too expensive 4x XLI 1500 is absolutely usable and vastly bettert than the 7 chanel HT amp would have been
Continuing on my currrent path of suggesting not Toid’s deisgns (haha) for a DIY subwoofer have a look at https://www.eighteensound.it/en/resources/suggested-designs/ They are a speaker company that have a few reference enclousre designs available for free. That may give you a bit more flexibilty on the sub design if driver availabilty for Toids is a problem. Id say 2 of the 18sound single 18 subs should be enough for the room size