How to judge a preamp's sound

The idea that a preamp should not have a sound is over simplifying its role.

No not really, they are a left over piece of equipment from the dinosaur days of vinyl, when there was only "millivolts" of output from cartridges,
today there are "volts"!!! all that's needed IF you have a few sources is a quality source switch box like this https://goldpt.com/sw4.html

And also back then in those vinyl days it was a long held saying as well, "the best sounding preamp should sound like a piece of wire" or "sound like no preamp"

Cheers George 

i think we are making this too hard

to figure out how one new piece of equipment sounds, just insert it into a ’reference’ system that the owner knows very very well and hear the SQ delta (if any), substituting for the reference unit ...

in this case, discussing a preamp, insert pre X in place of reference pre A (and reference pre A may indeed be no preamp at all)

then, to confirm, if you want to be more sure, try against a second reference pre B... if you wonder if there is pre to power amp matching issue, try a second reference power amp

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speaking in ideals or in theory is just that... in the real world everything has an impact on the sound

and we also don’t know what the signal actually sounds like, since there are various equipment and transducers capturing, processing and reproducing, none of which are ’ideal’, and most far from it

And also back then in those vinyl days it was a long held saying as well, "the best sounding preamp should sound like a piece of wire" or "sound like no preamp"

These two, 'sounding like a piece of wire' and sounding 'like no preamp' are not the same. The latter is good, the former is really variable!

I've always been intrigued by "sound(ing) like a piece of wire." Do we have wire listeners???

back then in those vinyl days it was a long held saying as well, "the best sounding preamp should sound like a piece of wire"

These two, 'sounding like a piece of wire' and sounding 'like no preamp' are not the same. The latter is good, the former is really variable!

I’ve always been intrigued by "sound(ing) like a piece of wire." Do we have wire listeners???

If anyone questions the implication of this classic statement that goes back to gramophone days, then sorry, there is no hope, "get out of the game" another saying, just in case😉