It might be time to get that feature request ready.  This type of ‘Mutron Down’ idea has come up before.

There are the Octaver filters, if you want to crack the rafters.  That will double your bass down another 1 & 2 octaves.

For the time being, you might want to try out HotSawz.  Fast Attack; slower Decay, over the resonant filter, with a sawtooth voice (or six).  I use that almost like parallel compression, with dry alongside the generated synth voice.

That might work out well with a (monophonic) bass line.  I do find it to be very forgiving with perfect intervals, though.  With the occasional glitch when HotSawz is trying to ‘find’ the fundamental pitch.