It's overly complicated by the fact that it was intended to mount multiple sirens and had "mute" buttons on the sides so you could run both sirens and cut one out to compare them. If you removed the dual siren and mute options it would be much more simple. The resistors are over-built and have heat fins as well as radiate heat through the housing so you could play with this demo unit a lot at trade shows. For a single siren you really don't need all these parts or such a beefy resistor. You really just need resistors and speakers of the right capacity and rating.
This thread covers it well
I have a BP100 that I'm going to keep around the shop to test sirens on. I want to reduce the output. Is there something commercially made that I can put inline on the speaker wire to do this? Or can someone tell me how to make something to do it? I recall an install on an engine where they...
elightbars.org
However at the end of the day actually hooking up the speaker and recording the siren outdoors (if you have the open space) might sound the most realistic.