In a pinch one could substitute a consumer grade autotransformer in-wall volume controls for 25V speaker transformers. Years ago, long before one could purchase cost effective in-wall controls, I would make my own using a rotary selector switch and transformer. "In-wall" was a misnomer for my controls because the transformers were too large to fit in a wall-box. The guys at the parts dealership thought I was crazy because I bought those transformers by the carton.