How to judge a preamp's sound

The idea that a preamp should not have a sound is over simplifying its role.  
Ideally it should not add or subtract from the tonal spectrum and balance of the source but this is not realistic.  Preamps can be evaluated on their tonal presentation and balance and how well the do at the frequency extremes and how transparent and clean sounding it is.  
Also it should not add any additional noise or distortion but again this is not realistic. 
Finally the amplification withing the preamp can be evaluated on its representation of drive and dynamic energy.  For me a preamp should offer exceptional micro and macro dynamic energy.  
Bottom line- a preamp can easily be evaluated on its sound quality when compared against others.  Saying it should have no sound is a gross over simplification and not possible.  

If you’re trying to decide on a sports car, do you drive one car on a bunch of different roads to decide which one you like or would you compare several cars on the same roads? Sure, in the first case you’ll get a good idea of what that one car can do, but it’s not as helpful in providing a relative perspective and/or making a buying decision. And after all, isn’t that the most important information a review can provide to a reader? Reviewing one preamp with a bunch of amps is interesting and somewhat helpful, but IMHO it’s less effective than comparing one or more preamps to each other. Humans are inherently poor at judging things on an absolute basis and much better at judging relative performance. Every reviewer has had the experience of thinking they have the sound of a component nailed, then they substitute in a competitive product and they realize how off they actually were in their initial impressions. Without a comparison they’d have been way off in their assessment. This is why I always take reviews written without a comparison example with a HUGE grain of salt. Anyway, just my take on it.

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!