The impedance issue can be more involved than simply verifying a 1:10 ratio (which many consider a minimum with a more desirable number being 1:20 or higher). For example, many preamps (and particularly tubed preamps) have output impedance that is significantly higher at frequency extremes (particularly at bass frequencies) than at 1K Hz which is the typical frequency where the output impedance is reported. In those cases, the reported output impedance (i.e., at 1K ohms) can result in one ratio while the actual ratio at frequency extremes could be much different (i.e., lower). JA at Stereophile used to regularly report about this in his measurements of (mostly tubed) preamps. Below is an example from his review of the Lamm LL2 Deluxe:
The LL2's output impedance was a low 245 ohms in the midrange and treble, this rising to 3.3k ohms at 20Hz, presumably due to the finite size of the output coupling capacitor. As a result, the preamp's frequency response suffered from a premature low-frequency rolloff into the low 600-ohm load (fig.1, lower pair of traces). Into the higher 100k-ohm load (fig.1, top traces), the response is flat from 10Hz to 50kHz and just 1dB down at 200kHz.
It also becomes important with passive volume controls (i.e., passive linestage) where the output impedance is dependent on the impedance value of the volume control, and where the IC cable impedance becomes more important without active circuitry (which is why most recommend very short cables when using passives).
Regardless of what Paul says in the video, there are still solid state amplifiers with input impedance as low as 10K ohms. This is rarely a problem when using active solid state preamps, which typically have output impedances in the range of 100-200K ohms, or lower. However, amplifiers having low input impedances will not likely reach their full sonic potential when driven through a passive line stage. For this reason, the sonic impact of using a buffer (i.e., active circuitry to address output impedance) between a passive preamp and an amplifier can vary significantly depending on the differences in input impedance between amplifiers.
