Dodgealum, first let me digress for a moment and compliment your very fine system. I would especially commend your taste in speakers, phono stages, and cartridges :-)
But to address your question: The issue that most commonly arises when using a tube preamp (which often have relatively high output impedance) in conjunction with a solid state power amp (which often have relatively low input impedance) is that most tube preamps have coupling capacitors at their outputs which result in their output impedance rising to much higher values in the bottom octave or two than their output impedance at higher frequencies. The interaction of that output impedance variation with the relatively low input impedance of many solid state amps will result in some cases in perceptible rolloff of the bottom octave or two.
For example, John Atkinson’s measurements in the Stereophile review of the Calypso indicate that the output impedance of that preamp’s balanced outputs (which are presumably the outputs Lalit would want to use) is "112 ohms at 1kHz and above, this increasing to 3900 ohms at 20Hz due to the finite physical size of the output coupling capacitors." The interaction of the combination of that unusually wide variation of impedance as a function of frequency (unusually wide even for a tube preamp), and the very high 3900 ohm value that is reached at 20 Hz, and a 15K amplifier input impedance certainly figures to be problematical, especially if the speakers (or sub, if present and not driven from a **separately buffered** preamp output) are capable of reproducing a good deal of the bottom octave.
A high output impedance will become somewhat less critical of a factor as the amount of its variation over the frequency range becomes less. And it may not matter at all if the variation is relatively small. But to assure optimum compatibility, the rule of thumb guideline (as properly stated IMO, which it often is not) is that the input impedance of the destination component should be 10 or more times greater than the output impedance of the component providing the signal, **at the audible frequency for which that output impedance is highest.** And if only a nominal output impedance is known, such as at 1 kHz, and there is reason to suspect that the component uses a coupling capacitor at its output (which most tube preamps do, Atmasphere’s designs being a notable exception), a much higher ratio should be applied to the nominal value, such as 50 or even 75x IMO.
The output impedance of your Herron preamp is apparently specified only at 1 kHz, but that value is fairly low at 100 ohms. 100 ohms is 150x smaller than 15K, and therefore whatever impedance it may have in the bottom octave is unlikely to be high enough to be an issue with your KWA 150SE.
Best regards,
-- Al
