You certainly don't need an A-199 in order to integrate regular audio into your system. You can just connect the output of any radio, hi-fi, etc to the input of, say, your filter module or reverb or whatever directly. (Be careful with the volume!) I find the A-183-3 helps get the levels right, depending on what you're hooking up, but even then you may not need it.

Experiment first, buy more modules only when that doesn't help. :)

Hope that helps,
Zoë.