'My question is whether by doing this I lose some of the M3s control options - the S switches, ribbon and XY joystick. If I get a controller with an XY joystick can I definitely map that to the M3M and a coupl of buttons to the S switches? Could I use a third party iPhone app to replace the ribbon??'
You could recreate the M3's interface with another controller. I use my novation sl to map the buttons and switches. One thing to keep in mind is the buttons and switches transmit certain (sometimes different) CC#'s for different programs. Just because switch 1 may be CC#81, for example, on 1 program doesn't mean it will be mapped to that for another program..... I don't know anything about iphone apps to use.
'My second question is whether the master keyboards knobs and sliders can overcome the interface limitations of the virus snow - can I map most virus synth parameters to external midi controllers?'
I have the virus C and you can map *most* synth parameters easily. However, the virus c doesn't always use CC #'s to control the parameters. Things like filter cutoff/resonance, etc are CC#'s, but some are controlled by another type of message which escapes me right now....
'Does anyone foresee problems with having two modules running through one master keyboard?'
Perhaps.... let me give a practical example/problem. I have a yamaha ex5 which I use as a master keyboard. I also have the M3 rack and Virus C rack. I wanted to use the M3's sequencer to run everything. So, I had yammie midi out to M3 midi in, M3 midi out to Virus midi in. When you press a key on the yamaha to try to make the virus sound, no information was getting sent. That is because the Korg 'swallows' the midi information and doesn't echo or 'pass through' information received at the midi in to the midi out. Other keyboards may work like this, but my command station didn't, so I just wanted to point this out.
Anyhow, depending on your setup and your plans, you may need a midi patchbay or some type of midi merger. Saying all this, you may be able to overcome this particular problem by buying the keybed and M3 module together. I don't have the original keybed for the M3, but maybe the same problem would exist then?
If you want more M3 help, a number of member over at harmony central have M3's. A few of them are here at GS as well, just in the electronic instruments forum more often......