at the risk of being labelled an elitist, why is speaker selection so critical if recordings feature mainly amplified instruments ?
Hang on...acoustic instruments all sound different too!
How can you judge what is real timbre anymore than with amplified music?
Sabian, for example, hand make cymbals...each one probably sounds different.
Sir Christopher Hogwood and the Academy of Ancient Music had period instruments copied/rebuilt so they could replicate the timbre of the day.
Pavorotti caried throat lozenges in his pocket handkerchief that he used to wipe his brow..surely that affected the sound outcome too
Perhaps the quickest/easiest test for timbre are male and female vocals so may be you have a point about unamplified... but even this is not perfect (I'll grant you that at least we are all skilled at recognizing voices)
So how to decide, for me it is easy. I tend to trust others who have ample opportunity for direct comparisons to "real" sounds.
My rsuggestion would be to audition a pro main monitor that studios use to impress clients with. If clients can be impressed by listening to themselves in a studio (just after a recording session) then the speaker/system must be doing something right in terms of accuracy. There are many speakers to choose from in this category and most are very good over a wide range of music. Although even in this case, microphone selection and placement can do a lot to influence the sound. One Nashville studio claims to have $1 Million invested in microphones alone...go figure!
Or like J. Gordon Holt...he selected a speaker that made his own recordings sound the most like the live event itself.
At the end of the day, I'd suggest that all sounds (amplified or not) must be all weighed and accounted for in the speaker selection. However, something like Sheffield Drum Track is a good starting point; great eliminator disc, as most systems fail that one altogether, and if a speaker can't do transients convincingly then how can any system connected to it begin to even hope to get timbre right...
So my quick check would be Sheffield Drum track followed by all kinds of vocals, and then loads of different instruments.