Even if size isn't a problem, running under cutoff always is. If you have a lot of source material with 20-30Hz content, I'd rather have a 85dB/W/M sub tuned to 20Hz than a 100dB/W/M sub tuned to 40Hz with a heavy bump of EQ at 20Hz. The latter approach sounds heavy and thick because anything under 40Hz sent to it makes pure distortion.

40Hz cutoff isn't the problem. Most music has very little content below 40Hz. But let the woofer rolloff without EQ. It will be much more natural sounding than to try and bring up the 20Hz level to match. If you want 20Hz, use a bass bin tuned to 20Hz. Don't try and push one designed for higher cutoff, whether it was made for prosound use or not. Higher power handling won't help if it is used out of band where the cone isn't loaded and excursion goes through the roof.