I currently have two 800W 12" Subs running from a 580W 4 channel bridgable amp, obviously i have bridged the 4 terminals to give 1 feed for each sub. I want more room in the boot and want to ditch one Sub and run a single 12" sub, i can switch the amp feed so that it recieved a MONO signal from the headunit, could i then run the 2 bridged sub cables to a single sub?
I can set all the channels to have the same level of Gain on the LPF.
I dont see a problem, perviously the 2 subs were underpowered from the amp getting only 240W each this way one will be getting more or less what it should.

 

Your amp should be more then capable of running the one sun on its own in bridge format. Can always turn your gain down a little anyway.

 

to clarify you intend to bridge a 4ch amp to 2ch and feed a single sub ?
If the sub is dual voice coil then fine as long as you DM the levels , if its a single coil then no you are looking at probs.
You will really struggle to match the levels correctly unless you have acess to a true rms volt meter.
The easiest and most benficial method would be to run the 2 subs isobaric style !!!! this way you get WAY WAY more bottom end reinforcement and dont need to increase your box size. It is by far the best method for running lower powered subs to gain maximum effect.
PM mr if you want more info on going iso with your subs.