An ignorant question about impedance

The ratio of 10 to 1, for amplifier input impedance to preamplifier output impedance is mostly looked at as a minimum acceptable limit. You hear about this mostly being a problem when matching tube preamps to low input impedance SS power amps. Many SS power amps have input impedance in the range of 50K to 100K ohms, and generally have no significant matching problems. However some amps, such as my McCormack DNA500, have a lower input impedance of 10K ohms. The other consideration that compounds the problem Aball already pointed out (low bass roll off) is that the preamp output impedance is often reported at 1K Hz, where it can be relatively low, but then the output impedance (that they don't report) increases significantly in the lower bass frequency range, due to limitations of the coupling capacitor size. I looked at some tube units with output impedance reported in the 200-400 ohm range, that when tested increased to around 4,000 ohm +/-. This could easily cause bass issues when using reasonably full range speakers. However, if you are using monitor speakers with limited output below 40Hz, that may not be a consideration for you. Those type issues are why you cannot simply put together a system of Class A rated components and expect to have good sound (there are other issues with reviews and ratings, but that is for another post!). BTW, impedance matching issues also sometimes arise when matching source output to preamp input.

Linearity is best preserved with low output impedance and high input impedance over the audible frequency range.

The impact of interconnects, speaker cables and speaker impedance variation with frequency should all be minimized in this case.

10 to 1 is a ratio at which effects barely begin to become audible (changing a cable, or interconnect or driving a speaker with a variable impedance load)

There are other factors too, including some very good points made by others already on this thread...but this is the essence.

Thanks so much for your clear and thoughtful responses. Back in college (in the 1970s) when I was spending a bit of time studying English literature and journalism (and a lot of time drinking beer) I used to wonder what those guys over in the engineering building were up to. I'm betting now that some of them were learning about impedance and all that kinda stuff. I've got some reading (and listening) to do. Maybe, just maybe, I'll ask one o' these days about cartridge loading. Thanks, again.
You guys are all right. -- Howard