As Ralph Karsten (Atma-Sphere) and Roger Modjeski (Music Reference) will tell you, the lower the impedance tap on a tube amp, the lower the output impedance (higher damping factor) and distortion, generally speaking. Unfortunately, that comes with less power, except in the case of the unique Music Reference RM-200, which produces more power at lower impedances.
Modjeski recommends using the lowest impedance tap possible that provides the power required, as the lower impedance taps drive the output the tubes the least/softest (they will therefore last longer), and with the least distortion. Plus the lower output impedance is really a significant benefit, allowing the amp to produce the flattest frequency response it is capable of.