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.
An ignorant question about impedance
I've read over these many years about the importance of properly matching a preamplifier's output impedance with an amplifier's input impedance. I've always just accepted this as something that has got to be dealt with by folks who understand such things. But now I'm wondering: What happens when there is a mismatch? Is there some threat to the equipment, or does the reproduction of music suffer in some obviously discernable fashion? (Highs or lows or mids are like this or that or some such?) I recognize that this is a boneheaded query, but I've found myself wondering and I've not been able to find the answer through my online searches. If someone could point me in the right direction, my non-technical mind would rest easier. Thanks.