vimsynth/modules/sub
A sub-oscillator an octave (or two) below whatever audio you feed it. It TRACKS the waveform rather than having a pitch of its own, so it cannot drift out of tune with the oscillator above it, ever. Patch a VCO into it and mix its output back in to thicken the bottom.

SUB to get this card.Down the left edge of the card, top to bottom.
Down the right edge, in the same order.
Left to right across the face — the dials in the picture.
sqr is the classic sub — solid and simple.The full reference — every parameter curve, the patching notes and the worked examples — lives in the manual: docs.vimsynth.com/modules/sub. In the app itself, :help SUB opens the same page without leaving the rack.