The majority of tube-based preamps, including the LS-7, have coupling capacitors at their outputs. The impedance presented by a capacitor rises as frequency decreases, which can result in the output impedance of the preamp being much higher at deep bass frequencies than its specified value, the specified value usually being based on the output impedance at a mid-range frequency such as 1 kHz.
So if the input impedance of the amp is not high enough, and keep in mind that for the LS-7 ARC recommends 20K as a minimum with 60K being preferred, the interaction of that input impedance with the rise in the preamp’s output impedance at low frequencies will result in weak bass. Depending on how the impedances of the specific designs vary at other frequencies there might also be adverse sonic effects at higher frequencies, but their sonic character won’t have much if any predictability since those effects will be dependent on the specific designs.
The commonly cited rule of thumb to assure impedance compatibility for a line-level interface is a 10x minimum ratio between the input impedance of the component receiving the signal and the output impedance of the component providing the signal. However, what is often overlooked is that the 10x ratio should be applied at the frequency within the 20Hz to 20kHz audible frequency range at which output impedance is highest. And as I said manufacturer specs commonly just indicate a nominal output impedance that is presumably at 1kHz or some other mid-range frequency.
If the preamp or other component providing the signal has been reviewed by Stereophile, John Atkinson’s measurements usually indicate the worst case output impedance, i.e., the highest output impedance at any audible frequency. If only a nominal output impedance is known, though, IMO a minimum ratio of at least 50x, and ideally 75x, should be used, especially if the preamp may have a coupling capacitor at its output as most tube preamps and at least a few solid state preamps do.
All of which is not to say that there will necessarily be a problem if those ratios aren't satisfied; as I indicated it would depend on how the impedances of the specific designs vary as a function of frequency. But it is to say that satisfying those ratios assures that there won't be a problem.
Regards,
-- Al
