Which amp are you using? Most A/B circuitry amps are only 2ohm stable in stereo operation. By bridging the amp, it becomes 4ohm stable mono. Without knowing the model # I cant say for sure though.

Unfortunately, if its only 2ohm stable stereo, you dont have any good options.
Unbridging and running each parallel will give you 1ohm so you would have to run series for 4ohm load on each channel. To run the amp bridged, your only options are .5, 2 and 8ohm loads.

My 2 cents, start looking for class D <2ohm stable amp....