What would the sound become if the impedance drops to 15K balanced or 10K unbalanced?The rule of thumb for proper matching of preamp output impedance and power amp input impedance is that the power amp input impedance (or the combined impedance of two amp inputs, if they are driven by the same preamp output stage) should be at least ten times the output impedance of the preamp, at the frequency within the audible frequency range for which that output impedance is highest.
That requirement will be met by most (but not all) solid state preamps under most circumstances. However, many tube preamps have output impedances of a few hundred ohms at mid and high frequencies (which is also ok under most circumstances), but which rise substantially at deep bass frequencies, often reaching a few thousand ohms at 20Hz. That rise occurs because of the coupling capacitors that most tube preamps (and some solid state preamps) have between their output stage circuits and their output jacks. That rise in output impedance at low frequencies will roll off the deep bass significantly when connected to a load impedance as low as 10K or 15K.
If the 10x rule is not satisfied, frequency response irregularities can also occur in other parts of the spectrum, to the extent that the impedances vary as a function of frequency. Under extreme circumstances, distortion can also worsen significantly.
If the preamp has been reviewed by Stereophile, John Atkinson's measurements which are usually included with the review will indicate the preamp's worst case output impedance, and whether or not this may be an issue.
Regards,
-- Al