so what would the difference be if u had 1 fi 18btl fully loaded daily running 4000watts rms or if you had 2 18bl's loaded running 4000watts i would imagine that the 2 bl's would be louder cause more air movement? i dunno i just got done reading the efficency post that was like 8 pages about 500watts to the btl or for that matter what db increases would you be looking at it you ran the btl18 at 2000watts rms vs 4000watts rms what are the sound characteristics between the btl and the bl ?

if the recommended box size is 5-10 cubic feet does that mean that the 10 cu ft will hit lower take less power to get louder but can blow easier? and what would the difference be between 6cu ft tuned to 32hz and a 10 cu ft tuned to 32hz maybe the bigger the box the poopier the sound quality?

I wouldn't run a BTL on 4000 watts daily. They are great woofers, but very few can take twice their RMS. Not only that, but the two 18s would be louder not only because of the increased surface area/displacement, but because the woofers will have less power compression.

the 10 cubic foot box will be more efficient, play lower, and have a more level frequency response graph. The smaller box will have more of a peak above tuning.

As for louder, that will depend more on your box and installation, but I'd also agree that running 4kW daily is ridiculous.