Gotcha, I guess I would have to measure the input impedance, since the SSR board doesn't specify that. Any thoughts on the fuse? Since I only drive one channel at a time, I thought that might be simpler.

Thanks Henry. I was wondering about coupling the inputs capactively. Is the intention above that all the components above would be connected to the input of the SSR, or does M1 represent the FET at the input of the SSR?

That's a good question, but one I'm not sure I can easily answer. The solenoids are in old, hard-to-find, pinball "score reels", so I'd rather not test to failure. I have already burnt one up, but I don't know how long the fault was present for.