The problem with subwoofers and planar speaker does not have anything to do with boomy response. The different radiation patterns create a very slight discontinuity where the sound is crossed over to the subwoofer(s). You will only be able to hear this if you listen to some panels with a very good low and and the same panels augmented with a subwoofer. There is not a difference in the quantity of bass or the extension. There is a difference in the integration of the bass. With the sub(s) the bass does not seem to come from the same place. Without the sub(s) there is a seamless coherency that no other speakers seem to have.
If there is a boomy sound from a panel sub combination it is almost always caused by either too high a crossover frequency, too high a level on the sub(s) or both.