If your store has a service department they should have Yamaha's CD of schematic diagrams on hand. Check the audio output circuitry and you'll find that the mono plug is not a true summation of both the left and right outputs. Electronically, the mono output is a pseudo mono, at least up until the Tyros1. I don't have schematics for the T2 and T3 so I do not know if this was rectified with the introduction of those two models. Unfortunately, I no longer have access to a digital DB/SU meter to determine if the sound is fractionally louder, but I seriously doubt if this is the case--or at least common sense tells me that if the volume controls on both channels are identical and the master volume is unchanged, then plugging in an additional instrument at the same volume into the mixer will not change the master output.
Deane, you may want to put a call in to Bose's service technical department and they'll be able to provide with the information you need about the tower connections. They have an outstanding service division and they can quickly provide you with an answer. I have encountered venues where ceiling height was a problem, namely a 100-year-old American Legion lounge in Havre de Grace, Maryland. The ceiling was just 7-feet 3-inches high, which meant I had to lift a ceiling tile to insert the tower's top section. When I did a load of dust came down from the ceiling, which was a real mess.