How much off-axis are you sitting? An alternative or addition to the balance control (which can mitigate it up to a point), is to toe in the speakers so they are pointing pretty much directly at the original sweetspot. Somewhat counterintuitively, more toe-in can give you a wider sweet spot.
Why does this happen?
If your speakers are toed in towards the sweet spot, and you move say to the left, you will actually move off-axis from your left speaker as well (getting less energy from it despite moving towards it), and still get quite a lot of energy from the right speaker as it is pointed towards you.
If your speakers are toed in less, and you again move to the left, you will actually move closer to the on-axis of your left speaker, increasing the energy coming from that, and you will move further off-axis from the right speaker, as it wasn't pointing towards you in the first place. So you will quickly experience an imbalance overemphasising the left channel, more so than in the first example.