What happens when you pair a low ohm amp ( Manley Stingray ipod ) with a high ohm speaker ( Zu Oman/ Spacial M3 or 4 Turbo triode) ? 5 Ohm amp output to 12 to 14 ohm speaker.
Short answer: nothing. Works fine.
Long answer: Tube amps sometimes have different outputs because of the way they work. They will sometimes have 4 and 8 ohm terminals. Internally these come off different taps of the same transformers. Even in that case you can still run either way, its just that you might notice it sounds a little better one way or the other.
Where problems usually come in is low impedance speakers, because they can result in current draw the amp is unable to deliver. High impedance speakers are like anything else high impedance, very easy to drive. Think inputs to amps, pre-amps, etc that are all in the thousands of ohms. Its the low impedance inputs where you run into compatibility problems.
Keep in mind that in any case speaker impedance is nominal. In other words your speaker is not 12 ohms, its something that varies with frequency, averaging around 12 ohms. But at any given frequency it could be a lot lower. This is the main reason people say to match your amp to your speaker, because if the speaker impedance varies a lot and the amp is poor at remaining stable in that same range you can wind up with problems you can actually hear. Which is after all the bottom line. So you hook it up and if it sounds fine then revert back to the short answer.