[Sleeves]

I've got the old master kit hanging out, but the 24 dB/octave modules only go down to 50Hz in pre-built. I do have blanks available, and according to the document I'll link below momentarily, you'll need to solder in 8 470KΩ resistors to get 15Hz. There's also a formula on the document you can use to achieve a lower frequency is 15 isn't low enough for you.

Note 1005 (audiocontrol.com)