I had the same problem with my subwoofer unit. I found the zener diode ZD3 was fried. The pic below shows the diode and its clearly tagged ZD3. You need a multimeter to check this before you order anything. This diode should only transmit power in one direction. Its size and spec is 18v .5w zener diode. (that's 1/2 w) It's very inexpensive and the whole repair took about an hour. This is from a Boston Acoustics subwoofer MCS 160
Ok..the procedure is easy.
1) unplug the unit and bring it to a well lit work area.
2) remove the panel from the back and locate the board that is held to the panel by 4 screws and has the ZD3 diode. The pic below shows the diode at the end of my test lead. Note the direction of this part and reinstall the new one in the same direction. Its a simple de-solder and re-solder project. I elevated the new diode about 1/8 " from the board with a small strip of copper. This acted as a heat sink during the soldering and elevated the part above the board to help it dissipate heat later when in use.
3) re-install the board and plug in all the connections.
4) this worked for me and I'm enjoying my sound system again!..good luck