A point that should be kept in mind is that the commonly stated 10x guideline for optimal impedance matching of line-level interfaces is commonly mistated and misinterpreted, as I see it.
That guideline says that for line-level interfaces ideally the load impedance should be 10 or more times greater than the source impedance, at the frequency for which that ratio is lowest.
That is commonly misinterpreted to mean that if the ratio is less than a factor of 10 sonics will necessarily be compromised. Which is not correct. Meeting the 10x guideline will pretty much assure that there won't be an impedance compatibility problem. But not meeting that guideline does not necessarily mean there will be a problem. It depends mainly (although not entirely) on how the two impedances VARY as a function of frequency.
If the two impedances are essentially resistive, and therefore do not vary significantly as a function of frequency, and if the load impedance is not so low that it causes a significant degradation of the performance of the output stage of the source, such as a rise in distortion, and if the high source impedance does not result in excessive interaction with cable parameters, especially capacitance, then even a 1:1 ratio would be fine. The only effect would be a small reduction in signal level, which would be compensated for with the volume control if necessary.
Most (but not all) tube preamps have a substantial rise in their output impedance at deep bass frequencies, as a result of the output coupling capacitor most of them use. That will cause a significant rolloff of deep bass response if the load impedance is not substantially greater (ideally 10 or more times greater) than the source impedance at 20 Hz. Raising the output impedance of a solid state source by means of resistors or potentiometers will not cause that impedance variation, and will not have that effect. Although if the output impedance is raised to very high levels, such as the 30K number you mentioned, there will likely be significant rolloff of the upper treble, and consequent softening of high speed transients, resulting from the interaction of that impedance with cable capacitance.
A corollary to all of this is that a high degree of consistency should not be expected between the results of these kinds of experiments among different systems. How the impedances that are involved vary as a function of frequency will be different for different components. Interconnect cable parameters will also differ from system to system, as will the lengths of those cables upon which the parameters are dependent.
Regards,
-- Al
