Audio Systems: Combining speaker outputs into 1 signal, dual voice coils, denon pma

Question
I have a subwoofer which was driven by a plate amp. The amp died and I am now using an older Denon PMA-720 integrated amp to drive the speaker. I run the sub pre-amp out from my surround receiver into the Denon and use just the one speaker channel to drive the sub. Sounds pretty good but I'm not utilizing have of the Denon's power. Is there any way to use both speaker outputs? I don't see anything on the Denon that says it's bridgeable to mono.

Answer
You cannot bridge the amp and you cannot parallel the audio outputs.

If the subwoofer has dual voice coils you could drive each coil with one channel of the amp.

Otherwise, you can use only one channel of the amp.  

But, honestly, even if you could use both sides of the amp that would double the power, supposedly.  Doing so is an increase in power of 3db.  Such an increase is only just barely noticable to the human ear.  To get a notable increase in output you would need much more power to the speaker.

C