Another great thing to record is the human voice. We are all very familiar with that sound, and any "vowel" coloration (as J. Gordon Holt put it) will be glaringly obvious. WARNING: Most loudspeakers fail this demanding test. The QUAD ESL (introduced in 1957!) excelled in reproducing singing voices, setting a standard few other speakers to this day can match.You are right for what you say about this Quad speakers...
But i will add something: all other speakers need more help from tuning room acoustic controls and they could reach some new level.... For sure each type of speakers are different, but no speakers are perfect on ALL acoustic factors even Quad...
We never listen to speakers, we listen to acoustically controlled or uncontrolled room/speakers....
But some speakers speak to us better than others in the same room....but adapt the acoustical settings of the room for each type speaker in particular and even our taste could change or be less affirmative...
No speakers beat the room says a great acoustician....
But i will repeat what you just said: QUAD is one of the greatest speaker ever designed...Precisely for the reason you said already....