I'm running FW v1.37 and the lowest setting available is 40Hz @ 12dB/octave. From what SD says anything >6dB/octave is done in DSP. The 6dB/oct cut in the analog stage is inactive until one of the other frequency slopes is selected. There is no way to have just the analog filter alone be active at the moment. I have asked them to add this as a feature request.
So, you're down 12db @ 20Hz, what's wrong with that ?
You cannot hear it, only the harmonics.
Reproduction of 20Hz takes more than you think, then more than that.
Reproduction of the nasty harmonics of 20Hz is all too easy and that's potentially what you'll hear.
Pure 20Hz is really air and cone excusion anyhow.
Try this:
Rip your favorite Steeley Dan song with great tight bass that sounds good on any box you play it on.
Then look at a spectral plot of it in SF, WL, AA or whatever.
.....surprize !
Cheers and did I +T you on the SD already ?
Congrats !