This was exactly what i was going to say. radiorobwalker is exactly correct. If you want the best, you need to get the best. I build Police/fire vehicles for a living, and using systems designed for emergency and these products meantioned are some of the best in the industry, and are aviable for a reosonable cost.
I bought the whelen sytem listed above for around $140. It is both a PA, and air horn system. To be honest, i don't really need the air horn. Its just a bonus for for the PA for me. I'll still wire it up though, and tap in line of my horn switch so when the system is on, my factory horn will activate the air horn. (without turning on the factory horn).
I aslo have an old federal signal smart siren, but since it has the acctual siren tones, i don't want to take a chance. If i can find software for it, i might still use it. Could use it to controll all my lights as well, as well as the radio rebroadcast feature could be nice
Ging to order my CB next week, along with a bearcat scanner. Going to install everyting at once, so i'll post up some pics of siren speaker, mic clips etc.