Flat Anechoic Measured Frequency Response Speakers

The ATC has a very flat response. Do you find that flat responses sometimes limit the 3rd dimension to the soundstage? In other words, do you find it is more like a two dimensional picture than a three dimensional picture depth-wise?

No not at all. A flat "power response" without the usual treble and bass boost can sound extremely natural and is hard to distinguish from live or real instruments. The vast majority of speakers have a strong off axis response in the bass and treble with a dip in the midrange. Sometimes called the "BBC dip", this has become so popular in consumer speakers since the 80's that a flat power response is rare and is usually interpreted as "forward" or "harsh".

There is a strong correlation to soundstage (although most depends on the recording)...there is a tendency for instruments to sound closer as if they are in the room with you - so you could argue that there is less depth with this pattern - conversely you could argue that other speakers will never reach as forward into the room or close to the listener forever relagated to sounding as if you are at the first balcony....so I think the soundstage moves forwards somewhat but is no less or more compressed than speakers with a BBC dip.

According to Dr. Toole's group that has been researching subjective listening for the past 2 decades, it seems to be very important. Not necessarily flat, but smooth.

The not necessarily flat point is important. Supposedly people like a deviation from flatness in the mid-bass and on top, according to Paul Barton (who also worked with Toole).

The thing that doesn't impress me with their methods is that they cherry pick their testers and instruct them in what qualities they should listen for in the speakers before using them in the tests. They only keep the people who demonstrate an ability to pick out the qualities they are instructed to pick out for the tests.

Not necessarily flat, but smooth.

Agreed - exactly! Very few people are aware of this. When I talk about tone control adjustments audiophiles cringe ....but they should not. It is no crime to tweak in this way. In fact a ruler flat on axis response is NOT all that important to our listening pleasure...it is actually the off axis that matters most.....in fact the MOST important thing is that the on axis and off axis behave in at least a uniform way with consistent gentle roll off as you get to wider angles...this is what sounds natural!

What sounds unnatural to our ears is a speaker transducer that has either strong directivity or wobbles and bumps off axis such that the on axis and off axis curves do not all behave together in a smooth way - in many poor designs they often crisscross as the cone breaks up or there is a sharp discontinuity at the crossover region (suckout or sudden abrupt change in dispersion pattern)

See this discussion SM75-150S. You will see the author correctly emphasizes that it is NOT the smooth on axis frequency response that is so important in a driver but the smooth consistent behavior between on and off axis and a wide dispersion.

SMOOTH is the key ingredient. This is why many designers pay so much attention to smooth phase response which is a warning sign of unwanted things going on with the design.