i just joined this group having recently got a k1m
i see there have been sporadic discussions of adding filters to this
clearly an easy option is run it into the pc and use a free vst
such as classic filter
theres not that many cheap hardware options
i have just tried using a 'fat' oberheim string patch
ie a multi of it +itself one octave apart
with an alesis philtre in a set up linked to the other modfx units
(i have them all except the metavox) and got some pretty spectactular
sounding results (with other fx as reverb on the end)
the other option is to try more dj oriented stuff like a korg kaos
pad, alesis airfx or the discontinued stanton thing with the mouse
ball controller in it
or if you have some guitar pedals you could try it on an autowah