Just implemented Antti Huovilainen's 4-pole ladder filter (as described in http://dafx04.na.infn.it/WebProc/Proc/P_061.pdf, basic version without the extra tuning), and it sounds quite nice and smooth. No self-oscillation, though - hope that will come with the tuning. Fixed-point version, with hard saturation instead of tanh(), also operational and sounding good, but will require more tweaking.

Note, this is not tested on the device yet - only on the host system, but using the same basic audio engine in a different build configuration. There likely is a lot of room for improving sound and compute efficiency.