Using self contained siren with a siren driver

[Mike rock <mikepier@xxxxxxxxxxxxx>]

I have a Napco home security system.Recently I renovated the hallway
in my house and wanted to eliminate the ugly bullhorn siren that was
visible in my hallway to a more sleek looking siren. So I purchased
this:

 http://www.mgelectronics.com/productdetails.cfm?productid=104

 When I went to install it, I noticed only 5 VDC at the wires when
activating the siren, but it still worked, so I just left it. Now I
realize why I was getting only 5V out. Turns out I have a siren driver
in my panel, and my old bullhorn was actually a speaker only. And I
can't re-wire the speaker wires in the panel to the +12V bell output
because I have a second bullhorn in my attic by the gable vent thats
being fed(paralleled) by the same pair in the hallway ( It is spliced
somewhere in the attic). If I did that, the siren in the hallway would
work, but not the speaker in the attic. So now I'm not sure what I
should do. Should I just leave it since its working, or maybe bypass
the circuit board in the siren and just go directly to the speaker?

By the way,the driver is an Altronix ALSD2.

 Thanks again