Phil, I don't mean to say that all preamps necessarily sound alike, but only that the low output impedance of the CMC series amplifiers tends to remove one prominent cause of different sound with different preamps.
I'm looking for, but not finding at the moment, an impedance chart which was drawn a few years back by a friend of mine, based on measurements of a high-quality, U.S.-made figure-8 ribbon microphone. The microphone's output impedance is 300 Ohms throughout most of the audio band, but near its resonant frequency (between 50 and 90 Hz, depending on the individual sample), the output impedance shoots up to a value well above 1000 Ohms--literally "off the chart." If I can find this piece of paper soon, I'll scan it and attach it here.
What Ohm's Law says about such a microphone, when you plug it into a preamp with an input impedance of (say) 1000 Ohms, is that around the resonant frequency, more than half the signal will be dropped across the microphone's own output impedance, and will never reach the preamp. That means the response around the resonant frequency will be down more than 6 dB below where the published curves say it should be. Even a preamp with 2 kOhm input impedance would load down this microphone and cause an audible decrease in its low-frequency response, as compared with the essentially unloaded state in which a microphone's frequency response is normally measured.
This isn't the only possible reason for different preamps to sound different from one another, but it is a big one--and using microphones with low, non-reactive output impedance tends to reduce or eliminate it in general.
As for as an input architecture like that of the John Hardy preamps, running a 30-Ohm microphone into it will simply tip the top octave response up or down a little--I forget which, actually. It's not a large number in terms of dB; there's no risk of parasitics or oscillation. But the response will simply be flatter at the top if you throw the "Lo-Z" switch which is optionally available on the Hardy M2.