if your tube amp has 4, 8, and 16 ohm taps (typical in tube amps, though the RM-200 offers 2, 4, and 8 ohms), and you hook up an, say, 8 ohm speaker to the 4 ohm tap, the power available to the speaker will be less than it would be if connected to the 8 ohm tap. But, says Music References Roger Modjeski, a tube amp so employed will usually be producing not only less power, but also less distortion, and better sound. As Al mentioned, Roger calls this tactic "light loading". In addition to lower distortion, an additional benefit of using a lower impedance tap is that the amps output impedance will be lower---it will have a higher damping factor, and will interact less with the varying impedance characteristics of the speaker load, resulting in a more predictable frequency response.While generally true, a problem that can turn up when doing something like this is that the transformer can 'ring' if insufficiently loaded. In addition, with such a load, it will not be as flat across its bandwidth, as the transformer will tend to express less of its turns ratio and more of its inter-winding capacitance. The 'lighter' you load the transformer the more of a problem this becomes.
So the result, while possibly reducing distortion in the output tubes, will be to **increase** distortion from the transformer (ringing) and degrade the frequency response. Of course, if the amp employs negative feedback some of this will get sorted by that, but a problem with negative feedback is that while reducing distortion overall (in particular lower ordered harmonics), it actually **introduces** higher ordered harmonics that otherwise may not have been present at all! Its best not to give feedback too many places to screw up.