Am I understaning the effect of imped. matching?

Not really familiar with engineering or even physics so much�so please excuse my ignorance. I�d like to better understand how SOUND may be affected by impedance.
My understanding is as follows:
1.Amps are usually rated to operate at a flat imp. (say 8 Ohm)
2.Speakers have nominal imp. But that varies across their frequency spectrum (so a speaker with nominal imp of 8 may actually dip to 4 or go a bit above 8 Ohms)
3.It is always ideal to match amp to a speaker of the same nominal imp.
4.Ideally speaker should have relatively flat imp. across the frequency resp.
5.Tube amps can tolerate lower imp. speakers (say amp is rated at 8 and speakers at 6 � assuming no major dips across frequency), however it is not really good to hook up tube amp rated at 4 ohms to speakers rated at 8.
6.Main risk of mismatch is excessive stress on the amp � no major effect on the speakers.
7.Amp that is driven outside of its �comfort� will sound weak, with flat bass and run much hotter.

Am I even close? What are the REAL effects of mismatch? How would you be able to hear it so to speak?

Thanks!