Got this off a Steve Hoffman music page.
"A bit of a reverse situation is formed inside the amp when you connect a 4 ohm speaker (or any lower impedance) to the 8 ohm tap. The output tubes work harder in this arrangement and may not last as long. Again, you will not get the most efficient power transfer to the speakers but this time, the lower frequencies will be limited in power. So you can use taps as a sort of mild tone control in this way as long as the output transformers and output tubes and surrounding circuitry of the amplifier is of good quality and as long as you don't get too far away from the target impedance. In other words, a 2 ohm speaker on the 8 ohm tap is stretching it too far here as well. A good rule of thumb is to never be off by more than 100% of the value, so nothing below 4 ohms or above 16 ohms should be connected to the 8 ohm taps. look at it as a factor of two and anything more or less than that will be destructive to the amplifier. The speakers are not really effected here other than the limiting of their power bandwidth. If you are going to mismatch impedances, to avoid any amplifier damage, you really need to be careful of input levels as well as that is what drives the voltage swing inside the amplifier. If you crank up the preamp and get a lot of power output, you are going to see higher voltages inside of the amp. Nothing wrong with that as long as the amp is designed to handle it and you are operating it within its specifications but if you are mismatching its expected load and output, you can get into some problem areas. "
