Well, remember that guitar music is transcribed an octave higher than it sounds. Although it’s written as high C, 2nd string 1st fret on a standard tuned guitar is really middle C. Which means 2nd string open A is 110 hz. So, 3rd string open on a standard tuned bass (4 string) would be 55 hz. With a low bass string of F#, the 3rd fret would give an A at 27.5 hz.
In practice, I suspect a player with a bass with a low F# isn’t going to be playing much below the 4th or 5th fret anyway unless he’s tuning it.
Citing this, and working my way down from an F# @ 185 hz and repeatedly dividing by 2, I came up with a LOW F# at 23.125 hz. I submit that while you’re more likely to feel it than hear it, the bottom end of the audio spectrum is 20 hz. You’d probably want at least 15" speakers but 18" would be better. And I think you’d want a graphic EQ because you’ll need a lot of power down there that you wouldn’t need an octave or two higher if you want balanced volume. So you’d want a more powerful bass amp so you can turn the upper frequencies down a bit. Maybe you can manage that by just turning down the mid and treble tone knobs and leaving the bass up on 11?