What stringreen says is true for most speakers, none more so than planars. When you add a pair of subs to Magneplanar/Eminent Technology or ESLs, you remove the very low frequencies from the panels, decreasing their displacement and the resulting distortion (planars are very inefficient movers of air), resulting in better upper bass and lower-mids on up into the midrange itself. You also remove the low-frequencies in the signal going to the speaker's amp, decreasing IT'S distortion, and leaving more available power for the mids and highs.
With the quality of musical subs available these days (Rythmik, GR Research---their OB/Dipole especially!, SVS, Funk, Seaton, REL, JL Audio, Vandersteen) there is no reason not to.
