DSM Noisemaker OmniCabSim
A tweakable cabinet simulator for both guitar and bass. Original FSB thread and still unverified schematic available here. .
Cabsim
Runoffgroove Condor Cab Sim
This was an old request. You can find all info and schematic on Runoffgroove website here. And a version with an added Notch Depth switch.
MultiCab Simulator
Here is a very nice project by Dschwartz from the DIY Forum. You can find the original thread here 13/01/2017 Layout updated! Change 4n7 cap connected to Flat 1 to 47n.