In a ported box you gain output by using the port area to your advantage, the box makes the noise. Essentailly, the fact that your box has holes in it makes it louder a few hz above tuning, the backwave of the sub is able to combine with the front. However, if you go below tuning, the holes in your box (ports), become just that, holes in the box. The sub will act as if it's freeair and can easily bottom out, leading to mechanical failure. A common rule of thumb is to set the SSF 1/2 octave below tuning. IN your case 37/2=18.5 37-18.5 is approx 18hz. A tone that low would quite possibly kill your woofer (check out KB for more info:laugh: So try a mid 20's tone, maybe 25 if 30hz isnt' giving you what you want. Or just block your ports and turn the bisch up!
ps. 15hz tones free-air looks neat.