So... 20001bd's are on average 48% underrated according to the RF site. So that's 2960 rms at 1 ohm or 1110 rms at 4 ohm. The SX18 will be in 6.5cuft 70in of port at 32hz. What load should I run it at?
Figure that's almost triple the power, which would effectively be a (theoretical) ~4.5db gain....realistically, possibly around 3db gain in output. But also an exponentially higher risk of damaging the subwoofer.