The Sherman Filterbank is an awesome piece, and truly one of a kind. Any older analogue synthesizer with audio input can give you some interesting options. For stereo, link two Sherman's (Shermen?) or get some vintage modular with stereo competence.
Like, for instance, the EMS VCS3 "The Putney". It's stereo, and I just can't get enough of it. (Been sitting here listening to a bass note repeating and evolving for the last twenty minutes or so, I'm nuts)
Actually, I'm seriously considering getting the Sherman Filterbank to go with my Putney.
Cheers,
Tomas Danko