Mr Crowherst talks some about just such imballance and how to compensate for a lot of it with a simple resistor. Although your circuit is hooked up differently (still not sure exactly how, but I know you will tell us sonn enough) you may be able to apply similat techniques to compensating for any imballance.
One of my friends built a dual transformer output for a power amp without balancing it and it sounded awful. He didn't even try to fix it, just scrapped the project and went on to the next. They can sound good, but it takes some trial and error to get it right.