It's pretty rare that you CAN bridge a 2 channel amp. Unless the amp has a mono/bridge switch or jumpers, you can't. Most do not.
Can you bridge any 2 channel amp?
In the risk of sounding dumb, can you bridge any 2 channel amp? Can you not just twist the pair of negative outputs together and then the positive outputs together and then plug them into your speaker? I'm pretty sure you can't but I thought I'd ask the experts!