50' speaker cable is no cause for concern, if it's large gauge wire. Otherwise (especially if you're pushing a lot of power to the speakers), you'll start loosing the damping factor, and the bass will start sounding mushy. I found the cheapest way to make a high-capacity speaker cable is to buy a ...

The transformer is center-tapped... Never considered the ground loop aspect. I should be able to verify that theory if I hooked up some of the units to just one leg of the secondary. If no hum, then your analysis would be confirmed.

You would think that someone would manufacture a power supply ...

In our project studio, we have several pieces of gear that use 9 volt AC wall-wart type power supplies. (When you have 16+ wall-warts, I think the term "infestation" is applicable).

I thought it logical to assume that by purchasing a large current capacity transformer, I could replace the power ...