There are a couple of things we will need to know. The physical size and cost of the transformer is a function of the lowest frequency chosen . It is important not to choose a transformer frequency response better than the amplifier or speaker. By the way, most long distribution lines have capacity and develop coupling at high frequencies, so frequencies above 12,000 Hz may be seriously attenuated. This frequency response will determine the design parameters of the step up transformer and, of course, the cost.
Lets tackle the little question of phase.
All of the transformers must be phase correct or the system will be a sound mess. This applies to each of the step up and each of the step down transformers. Stick with one transformer manufacturer for the system and make sure you are supplied with a detailed bid or specification sheet on each transformer to be used on your project. All rights reserved.