I thought this Q/A from Richard was really interesting because it's the first time I've seen this addressed at length. All caps are Richard:
Per: Hello Richard. On the Stereophile measurement (in room) of the model Quatro the area between 100 hz and 400 hz are down about 5 db. Can you explain if it is realted to the speaker, the set-up between the room and the speaker or the set-up of the in-line filter between the preamplifer and the amplifier ? Best regards Per
HELLO PER, THIS IS PARTLY A NATURAL ACOUSTIC ARTIFACT AND PARTLY BAD ROOM SETUP. WHEN A SINGLE SOURCE LIKE VOICE OR PIANO IS LISTENED TO IN A ROOM THE FREQUENCY RESPONSE IS MODIFIED IN THIS RANGE BY CANCELLATION CAUSED BY FLOOR BOUNCE. THIS IS A NATURAL EFFECT. THE SAME THING HAPPENS WHEN A SPEAKER IS LISTENED TO IN A ROOM BUT IF YOU LOOK AT THE SPEAKER RESPONSE WITHOUT ROOM THE SPEAKER IS VERY FLAT THROUGH THIS REGION. IN THIS CASE THE EFFECT IS EXAGGERATED BY THE FACT THAT THE SPEAKERS WERE TOO FAR INTO THE ROOM. THE QUATRO IS DESIGNED TO WORK CLOSER TO THE WALL BECAUSE IT IS A BETTER AND MORE LIKELY AESTHETIC SOLUTION. THE ONLY WAY TO FILL THIS REGION IS TO USE MULTIPLE WOOFERS VERTICALLY ALIGNED ON THE FRONT BAFFLE SO THAT THE FLOOR BOUNCE IS AVERAGED OVER ALL OF THEM AT A SLIGHTLY DIFFERENT FREQUENCY. THIS IN MY OPINION CAUSES A HEAVINESS TO THE SOUND THAT MAKES THE SPEAKER SOUND LIKE A BOX. THIS IS NOT CAUSED BY MISADJUSTING THE HIGH-PASS BETWEEN THE PRE-AMP AND AMP. IF A LISTENER WANTED THE SPEAKERS FAR INTO THE ROOM THEY WOULD NEED TO BE PLACED VERY CAREFULLY OR TOO MUCH OF THIS EFFECT COULD BE A PROBLEM.
The original Stereophile in-room 1/3-octave response for the original Quatros looked like
In my own room, I get a broader dish out (speakers calibrated with the 11-band EQ)