In all fairness, you should expect a healthy dose of 'time to upgrade'. While Bose, deservedly, has a poor reputation for sound quality, I haven't heard much of their products failing like this. If the repair is $500 or more, which I would expect, then it would be cheaper to get a new system, and it would sound better for the money.
I don't think many people on these forums have the complete Acoustimas systems of this nature, which is why you may not hear much in the way of specifics to help you out.
The bass module in these Bose systems is basically the integrated amplifier. All audio runs into it, then it goes out to the speakers. If the internal amplifier fails, then you have to replace the entire module, which is just not cost effective.