I have a HAI Omni Pro II controller with a 10A111 two way voice module installed. There are two speakers attached to the module and I would like to add two more speakers. The documentation indicates that additional speakers can be added if they are wired in series. But it's my understanding that the volume would be diminished using this method. Thus, are there any other alternatives to add two more speakers to my system such as doing this with some type of amplifier in order to maintain the volume?