A lot of good advice, so far.
-Ghosthouse's advice of using a folding panel might be a way to reflect sound.
-Dekay's advice of using small adjustments to speaker position should also help.
Since you already notice an improvement from using 3 pillows, I think the advice erik-squires probably will probably give you the best result.
As far as 'more juice to the left', that isn't so. It is simply that you have one side reflecting more sound to your ears than the other side. 'Power' isn't a factor, just acoustics.
As I said before, if you can adjust balance, give it a try. Or, try moving the speakers incrementally.
If you want to nip the problem in the bud, then I believe going with acoustic treatments will provide you with the most effective means of resolving the problem.
Like most folk with high performance stereos, we don't have the ability to own a home with a room with perfect sound properties, so we have to either accept the deficiencies or try to compensate for them as best we can.
HTH
Bob