The Poly-800 is an easy mod target, due to it's single filter and inexpensive price. Source selection and level knobs can be added to modulate the filter cut-off by any source (i.e. DCO, noise, audio input, manual), rather than just the LFO and envelope. A switch can be added to control the filter slope (2 or 4 poles vs. factory 4 pole only). An audio input jack and accompanying switched trim pot can be added to one of the filter inputs. The filter resonance calibration pot can be replaced with a knob for manual control. However, a sysex router (e.g. MIDI Solutions MIDI Event Processor) can map a MIDI controller to filter cutoff and resonance (or any other sysex), without any modification. These mods are also compatible with the EX-800 and Poly-800II.
While the same mods as the Poly800 series can be made to the DW series (i.e. DW-6000, DW-8000, EX-8000), they are more complicated. A custom circuit board is needed to handle the required muxing, since several filters are involved. Note, if you like the DW sound, but want more waveform flexibility, consider buying a DSS-1.
Theoretically, the DW situation applies here, but we haven't tried modifying either of these. Technically, the filter switch modification wouldn't apply to the DSS-1, since it already supports both filter slopes. Additionally, resonance control may be added to the DSM-1, but once again, this would require a custom circuit board, which we haven't thoroughly investigated.
Contact our service department for a quote on performing any of the listed modification. The service department can also replace the battery (with a holder and new battery) and calibrate any of these synths.