2x BK Double Gems vs 1 BK P12-300SB-PR??

If you want the subs playing mono, yep, and you would on the stereo side ('B') use the miniDSP to sum it's output to mono too. Would be like this, purple being mono:
View attachment 1371366

However you can do stereo bass if you swap the position of the Sys and miniDSP.
I actually do it this way myself, but you have an extra step in that you need to also switch configs on the miniDSP based on what you're doing. Essentially run the miniDSP in a blank config to allow the AVR to do it's thing (although nothing stopping you from doing PEQ on the miniDSP still - and I have to because my AVR is old and has no room correction).

So,
Movie use = Sys in position A, Config 1/2 on miniDSP (1 could be blank, 2 could be bass boost)
Music use = Sys in position B, Config 3/4 on miniDSP (again depending how much bass you want).

I've also bought the WiFi module for the miniDSP so I can switch configs via the iOS/Android app on the fly. Which means I personally find it very easy to just click an input on a switch, and choose a respective DSP mode - but if you have family I can fully appreciate that sort of setup would be confusing and likely piss them off!!!

View attachment 1371370

The AVR you'd still want mono I guess and you just sum the input to both subs with in the miniDSP (just a bit messy to try and depict that on the diag).

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