So far I've made some simple IIR (biquad), FIR (for reverb mainly, with multiple moving window of 16 samples), and a special "linear filter" (can be used to emulate linear interpolation, mainly for control values in synths like volume). One of my current goals is to get hold of the filter algorithms used in the E-Mu Proteus 2000 series of synthesizers. All I know that later on, the Emulator X series also used the same synthesis engine, but for the PC, and the ability of running it at 192kHz.