Yes, Fred's solution is the best. If you don't want to spend the loot on the baluns, at least utilize all of the conductors in the CAT5 for maximum current carrying ability. Another solution would be to use 70 volt speakers and a 70 volt amp. The CAT5 wire would be fine for that application. But by the time you did that, it would be less costly to use the baluns and a discrete amp.