Passive speakers shouldn't have audible noise, so THD measurement is good enough. I have never seen measurements that show a speaker that can do better than -65 dB THD under any meaningful test condition. 100 years is a long time, I guess it might be possible.
The best speakers for THD... tend to have other constraints...
eg: Electrostatics - superb in terms of THD... which results in an ability to portray micro-detail superbly (yes I am biased, I do love stats!) - but then you get to an orchestral crescendo, or you play some "enthusiastic" rock... and the dynamic / SPL constraints can cause an issue (depending on what SPL you like to listen at).
Still, the setup I was happiest with, now sadly gone on the altar of domestic harmony, was an all electrostatic 4.1 setup (I was planning on adding another stat for center, but never got around to it). Quad ESL63's for surround, Quad ESL989 for fronts. - It was pretty awesome.
I do like my current Gallo's - but am aware that they don't quite match the stats... a compromise I have accepted - on the other hand they are capable of higher SPL's given an appropriate amp. - So I am accepting the deterioration in THD as part of my trade off. (when I first got the Gallo's I did do back to back comparisons).
And yes - I do look at the THD / SINAD of electronics before purchasing in some detail, but ultimately, am very aware, that the speakers will impose a greater THD in any case!
I have also done quite a few amplifier comparisons over the last 15 years in my search for optimisation ... and as long as the amp is running within its performance envelope... (ie: it can handle the speakers properly and can produce the needed SPL's into that set of speakers) - I have heard no substantiable difference between 60db SINAD and 80+db SINAD. In other words, at times there were what I perceived as differences, but they were not sufficiently consistent to be statistically significant - and might well have been confirmation biases.
Yes a little Tripath TA2020 chip amp, did sound worst...rated 13W @ 8ohm 10% THD (!).
It was also surprisingly OK at lower volumes... 0.03% THD 5W 8ohm...
So my expectation is minimum electronics SINAD = 60db... and I am unlikely to purchase anything with a SINAD below 70db.
Above 70db... other factors dominate... (it's also about value for money too - and achieving the best possible performance within a given budget - beyond a certain point pushing for more SINAD simply escalates the price, with little or no impact on audible performance!)
