Speaker volume control

i am looking for a way to limit volume between a Bose SoundTouch system and a pair of connected Yamaha speakers. At present, the volume is controlled from the SoundTouch app on PC/Tablet/Phone and a couple of times it jumped to max by itself. Bose does not have a solution for the problem.

Amazon US sells something like this but it costs over $100 by the time it gets here.