Great post. My progression with the dual 10” NVX subwoofers is as follows and am very happy with the system.
-Upgraded to 1K watt NVX amp, added toggle switch for remote turn on lead so I can disable when bass is not needed/wanted, and when I get out of car. So far no electrical issues in 6 months.
-Added poly fill to boost boxes, unknown impact
-Continue using factory sub and lowered crossover point on NVX amp to around 65 hz. Factory sub works well and NVX takes over the deeper lows. Use bassX on LOC to boost low end.
-Would love to try a different brand of sub but have not found one with identical specs to the NVX unit. Boost box has threaded inserts so it would have to match perfectly or new holes will need to be drilled. Would like to try a sub with a longer throw to see if it could get a bit deeper. Had JL Audio in my last vehicle and it may have gone deeper with similar power.
Very happy with how it sounds, but I am always tinkering and looking to improve.