I wrote some time ago that the Bose Soundtouch speakers cannot be used as a mini PA because even via cable aux-in they have a terrible delay.
Luckily, meanwhile there are several threads about this issue in the Bose user forums, as it is not only a problem for use as a keyboard monitor, but also for videos, PC games etc. So the issue has reached Bose, and I hope future models will get rid of the problem.