PFH, please keep in mind that a power amp is an absolutely stiff voltage supply up to the point where its overload protection is activated. Behringer specs show a damping factor of 300 or greater when used with an 8-ohm speaker. This means its actual output impedance is .027 ohms or less. Speaker designers assume this is the kind of signal source they'll be working with.

With an output impedance that low, the electrical interaction of paralleled speakers is essentially zero, just as the voltage on a good, stiff 120VAC circuit will not change when you plug in additional loads. Again, this only applies when the amp is working within its protection limits.

The exception is certain esoteric tube amps, which are sometimes deliberately designed with a comparatively high output impedance. One hopes live sound techs aren't using those.