Good find! It wasn't as matter of how careful I looked, but a matter of where I looked. I looked in the Polk manual, which did not provide the -3db 40Hz data.
In light of the new information on the Polk playing 40Hz at -3db, I would consider the JBL's -6db at 30Hz a better spec.
Is this a certainty? Absolutely not!
But how likely is it that the Polk would only roll off 3 additional db from 40Hz to 30Hz?
How likely is it that Polk would claim a low end of 35Hz, if it was only down 6db at 30Hz?
Thanks for the comment on the amplification. Looking back at my post, I see that it can reasonably be read with an emphasis on the differences in power between the JBL and POLK. My intent was to point out that the JBL simply has a pretty good amp for a $100 sub, period. I mean, these days, inexpensive amps typically give a power rating which is a 1kHz instantaneous powersweep at 1.0% distortion, and it is nice to see a specification of RMS watts with a specified bandwidth and a low 0.1% distortion level. To me, this reflects quality, both in the amplifier and in the company selecting the amplifier.
I should say that the -3db at 160Hz of the Polk is better than the -6db at 150Hz of the JBL. If the OP plans to cross this sub over at an unusually high frequency (say, if he has mini-cube satellites) this would be in the Polks favor. However for normal usage, this would not be a factor.