All the higher end PSRs contain a powerful synth similar to the MU128. You can't get at the features from the keyboard but via sysex you can. For instance, the PSR-2000 has 55 basic effects with up to 16 parameters per effect. Quite a number have LFOs where you can alter the frequency, amplitude, feedback, delay and depth. Most of them have variable filters of one kind or another. Just connect the PSR to your computer via MIDI and send it the sysex. There are several sysex editors out there that will do the job.