Alan, when I did my research to answer this question a couple of years ago, I discovered it is 28Hz for 6-string bass. I wanted to know because I didn't want to pay for deeper bass than I needed for my 2.1-channel room.
Randy, I am not the one blowing gear up, dude...LOL!
P.S. I can no longer find the reference that told me 28Hz. This reference says 31Hz for six-string bass...and 28Hz for piano and keyboards.