Hi - A few years ago, I had the Carvin unit & it's wired such that when you 'add' an extra speaker, it is in series, not in parallel-as most amps are-& thats why the volume is reduced-you are only drawing half the current when you add another speaker to these units. With most amps the extention speaker runs in parallel, you draw more current, but the volume of each speaker should be similar as if you were listening to one speaker.
I believe Carvin does this cause the amp 'normally' is running at max.
Cheers!
Gord