I just ran across one of these in my storage area as I was cleaning it out. The Symetrix 421 is a mono expander (noise limiter)/compressor (AGC)/limiter that I was using on AM with a Rangemaster.
Since I broadcast mono on FM for the increased range, I decided to fire it up and it worked very well indeed. I'm thinking of keeping it deployed, as the software audio processing I use takes a lot of CPU cycles (which eventually leads to problems on my less than modern day broadcast production computers - I generally have to reboot every few days).
I think the setup I'm aiming for is to run automation software (Zara) on one computer. I'll take the audio and run it through the 421. The audio will then be split - one feed to the transmitter, and the other to a 2nd computer. That 2nd computer will be running server software (Windows 2003 Server to be precise) so that it can support a large number of listeners (hey, I can dream, can't I?). The audio will be fed into Edcast to generate a stream, which will be served by IceCast.
While I've been threatening (to myself) to do this kind of thing for a while, it was the 421 that galvanized me into action.
Maybe next time I'm tempted to clean out something, I should go and read a good book.