Loss of bass when both left & right speakers are attached?

Recently, I noticed that the bass was almost completely absent so I thought the amplifier had blown or had an impedance problem. I have checked the amplifier (Rotel ) and found it to be ok. I then connected my accoustimass 5 to another amplifier to be sure and the same problem occured. I have removed the rear panel on the sub woofer and cannot see any visible damage to fuses or any other part but I am not an electrical engineer! Has anyone experienced this problem before or will I end up sending my speakers (which are way out of warranty) back to Bose?
Any help very much appreciated!