Right now, you have two 120 Ohm resistors across the jack (I'm assuming you have the black Neutrik jack, not the silver one). When you plug headphones in, the short that the jack creates across these resistors opens, and the level is attenuated a bit.
To alter this scenario, you can start by cutting out the 120 Ohm resistors. Now, when you plug your headphones in, you'll get no sound. Logically, one would then need to flip the output transformer connections over to the empty side of the jack, and leave the speaker connections as they are.
-PB