did you see that begin~ is "obsolete" according to the documentation? they tell you to use poly~ instead ;)
use "thispoly~" with the "mute message to control the active state of each voice… see helpfiles of thispoly~ and poly~.
post a patch if you need more help

Why not just nest [poly~]s for your waveform generator? Assuming you can only have one waveform selected at a time, just make a poly patch for each waveform type, and load them in dynamically.

For the X-over it’s a little more annoying, as loading in/out [poly] patchers dynamically can still cause some DSP stall, far as I know. But, if your control mechanism (ie, your check for 0) is outside of the nested X-Over poly, you could just send it a mute message.