The early reflections off the nearby walls on the corner-loaded speaker have the effect of increasing the loudness of that speaker.
Not much you can do about it in the bass region, but it is possible that aggressive use of absorption on those wall surfaces might make a worthwhile improvement at mid and treble frequencies.
That being said, using the balance control, if you have one, is the easiest and probably best solution, as absorptive material will also act somewhat as a tone control because it will be most effective at short wavelengths and becomes progressively less effective as we go down in frequency and the wavelengths get longer.
Duke