Threre are actually three things that can change when your amp's load that is not optimum:

    power
   
    frequency response

    feedback ratio

Many solid amps that draw feedback from the output will have much more feedback when loaded higher than optimum. Since feedback is used to get published distortion specs as low as possible, more feedback can cause instability or, at worst, oscillation, which can destroy speakers or the amp or both. Loading a solid state amp with higher than recommended loads is not adviseable unless you know the circuit.

Optimum load for both tube and solid state will give the most power and cause the output devices (solid state) to dissipate the least power possible.

Putting to low a load on a tube amp usually results in poorer frequency response which is important in very high quality stereo systems but not so important in a guitar amp.

As everyone has said, two 8 ohm speakers should paralleled and put on the 4 ohm tap. Performance is still affected by efficiency, since the more efficient unit will play louder than the other speaker.