So after all the discussion on the problem for existing nonspeaker DSP speaker owners the options on getting the AC12 to work with existing S leads are:
1) 511 slave patch ($150 US)
2) S lead to BNC adapter (M may have some-not sure of the cost)
3) cut off end of existing S lead and solder phono (RCA) plug and use a female phono to male BNC adapter.
There may be other options that I'm not aware of or that have not been mentioned.
It seems option 3 is the least expensive and for people that don't want to solder there is a solderless phono plug connector ( available at any Radioshack in the US - not sure about the UK) which should work although I haven't tried it.

Joe