Interesting experiment. Have you tried adding room treatment on the short side to "kill" early reflections?
My guess (not technical analysis) on the bass is that by adjusting the balance you do actually lose some of the low end output (balance adjustiment attenuates the signal on one channel), hence a reduction in bass.
My guess (not technical analysis) on the bass is that by adjusting the balance you do actually lose some of the low end output (balance adjustiment attenuates the signal on one channel), hence a reduction in bass.