The myth of having the impedance selector matching your speaker cab exactly is an old wives tale. Let me preface that by saying it is advisable to match the impedance of your speaker cab to the selector or output of the amp, but it is not necessarily imperative. Why though?
The output tubes have a range of load impedance in which they are designed to work within. It is broader than you would think. The transformer presents to the tubes a " primary impedance " ( load ) that is selected in order to make the amp operate a certain way. You can go lower in impedance in order to coax more output with higher headroom, or you can go with a higher primary impedance and have lower output with less headroom. The issue is the secondary impedance. The load ACTUALLY reflected onto the output tubes is the combination of the speaker's impedance combined with the output transformer. If you connect an 8ohm speaker to the 16ohm tap, the reflected load to the tubes is also cut in half. if you place a 16ohm speaker onto the 8ohm tap, the impedance reflected to the tubes is doubled.
So how much of a mismatch is bad? In my experience, a mismatch of one-half or double is 100% OK. I.E. you can connect an 8ohm speaker to either a 4 ohm or a 16ohm tap. You can connect a 16ohm speaker to the 8ohm tap, or you can connect a 4ohm speaker to the 8ohm tap. This amount of difference will never not more than double or halve the reflected load presented to the output tubes and would be within the acceptable range any given tube will operate within. If you venture beyond that 1/2 or X2 mismatch, then you get into the danger zone.
What is the effect on sound and output? When you reduce the primary impedance reflected onto an output tube, it is more easily able to drive that load ( it doesn't load down the tubes as much ), giving a boost in output and clean headroom. The downside is a tonal shift that tends to be brighter. less bassy and more direct or abrupt sounding ( less compressed ). Conversely, if you present a higher load to the output tubes, it reduces output and lowers headroom ( more load presented to the tubes to drive ), with a tonal shift that is darker, more bassy, compressed, and more distorted ( because of the lower headroom ). The selection of the primary impedance of the output transformer is selected in order to enhance the designer's objective goals. Do they want a louder, brighter, and less distorted output, or do they want a darker, quieter, and more distorted output?
You can shift the impedance selector or speaker load 1 selection either way to acquire a similar result. If you only have a 16ohm cab and you want to darken the amp up a little and acquire a little less headroom and output, you can put the speaker into the amp's 8ohm tap ( or speaker selector ). This is totally fine to do. Having an 8ohm cab offers you the largest number of options since you can shift it 1 generation high or low to acquire a tonal outcome.
As for running an amp with NO LOAD..... 100% not advisable for any amount of time. You may get away with it for a moment or perhaps longer, but there is NEVER any guarantee that the OT or the output tubes will survive if you start playing at appreciable levels with no load connected. Some amps have a small dummy load in them to protect them in such an instance, but there are not many that I know of that will tolerate such a situation for very long even if there is some level of protection.
