Again, without having knowledge of some or all of the system design, it is impossible to give an accurate possible solution to the originally stated question.

Autoformer volume controls are good at stabilizing the impedance seen by the amplifier, but in return, they absorb some of the output power from the amp which is lost in the form of heat. And autoformers of the commercially available variety are bandwidth limited.

Really it all depends on what you're trying to achieve.
If you're looking to create a level of sound close to reinforcement levels outdoors, then speakers and equipment of the "Home Entertainment" variety aren't going to cut it.

JJP