I won't opine on the wisdom of bridging your amp, and only point out that power to your speakers quadruples as does dissipation stress on the amp. Obviously 8 ohm speakers will be less challenging than 4.
Technically, modification should be relatively easy. No doubt you're familiar with the usual approach--- you modify the bridge amp to have a gain of -1 and drive it from the master channel. Connect a 33.2k (i.e. equal to R6) resistor from base of Q2 to the output of the driving channel. You should incorporate a 1uF series cap with the added resistor to avoid introducing offset error into the inverting stage. 33.2k and 1uf will form a 5Hz corner frequency. You should probably short the input jack with a sorting plug or wire.
Good luck and enjoy!
Technically, modification should be relatively easy. No doubt you're familiar with the usual approach--- you modify the bridge amp to have a gain of -1 and drive it from the master channel. Connect a 33.2k (i.e. equal to R6) resistor from base of Q2 to the output of the driving channel. You should incorporate a 1uF series cap with the added resistor to avoid introducing offset error into the inverting stage. 33.2k and 1uf will form a 5Hz corner frequency. You should probably short the input jack with a sorting plug or wire.
Good luck and enjoy!
I'm not sure how you would implement the paralleling--- seems complex and risky to me, but perhaps is simpler than I imagine. Bridging is pretty simple and low risk, and is easily reversed if needs evolve--- IMHO. But if you're seeking an interesting challenge, that's a different consideration. 
From the perspective of efficiency/stress on power supplies and heat sinks, I believe the two approaches are identical.
Have fun!
From the perspective of efficiency/stress on power supplies and heat sinks, I believe the two approaches are identical.
Have fun!