Audiokinesis is dead right on this one. My system is adjusted to be down 9 db at 20 kHz at 0 db. I would like at add one other variable to Audiokinesis's point and that is volume. As Fletcher and Munson noted the frequency response of our ears changes with volume. As volume increases we become more sensitive to low bass and treble. Or conversely as volume drops we become less sensitive to bass and treble.
This was the theory behind Loudness controls which boosted treble and bass for low volume listening. But, the curves were fixed and only accurate at one volume. Most of us never used them. We just turned the volume up until things sounded balanced. People gravitate to speakers that have a response curve that suites their taste at the volume they usually listen at, in the room they listen in. In this context 1 or 2 db here and there can mean quite a lot. I impulse tested a friend's Watt/Puppys several years back in the near field and sure enough both speakers had a 2 db depression centered at 3500 Hz. This is exactly where you put a notch filter if you want to cut out sibilance in female voices and is responsible for the "smoothness" that people found so attractive in these speakers.
As Audiokinesis mentioned, Flat sounds awful. The problem is that what sounds good is a moving target. Tone controls and loudness filters are severely compromised. They just are not flexible enough. The only way to deal with this is with digital frequency response control or room control or whatever you want to call it. And not just bass, full range. The speakers are impulse tested from the listening position. A frequency response curve is generated for each loudspeaker along with a correction filter that brings the response of the system to flat making both speakers exactly the same which is vitally important for imaging. You can view all these curves on your PC. Next you can overlay a response curve of your choice and design. You want your speakers to sound like Wilson Watt/Puppys? Just put a 2 db depression at 3500 Hz and you are good to go. If you are very fond of your speaker's house curve you can overlay that and you will have exactly the same sound you started with except with better imaging. You can have a series of curves for various purposes. The very best units have dynamic loudness control. The unit automatically shifts loudness curves with volume. The end result of this is that the music has exactly the same tonal balance regardless of volume which is very spooky when you first hear it.
This was the theory behind Loudness controls which boosted treble and bass for low volume listening. But, the curves were fixed and only accurate at one volume. Most of us never used them. We just turned the volume up until things sounded balanced. People gravitate to speakers that have a response curve that suites their taste at the volume they usually listen at, in the room they listen in. In this context 1 or 2 db here and there can mean quite a lot. I impulse tested a friend's Watt/Puppys several years back in the near field and sure enough both speakers had a 2 db depression centered at 3500 Hz. This is exactly where you put a notch filter if you want to cut out sibilance in female voices and is responsible for the "smoothness" that people found so attractive in these speakers.
As Audiokinesis mentioned, Flat sounds awful. The problem is that what sounds good is a moving target. Tone controls and loudness filters are severely compromised. They just are not flexible enough. The only way to deal with this is with digital frequency response control or room control or whatever you want to call it. And not just bass, full range. The speakers are impulse tested from the listening position. A frequency response curve is generated for each loudspeaker along with a correction filter that brings the response of the system to flat making both speakers exactly the same which is vitally important for imaging. You can view all these curves on your PC. Next you can overlay a response curve of your choice and design. You want your speakers to sound like Wilson Watt/Puppys? Just put a 2 db depression at 3500 Hz and you are good to go. If you are very fond of your speaker's house curve you can overlay that and you will have exactly the same sound you started with except with better imaging. You can have a series of curves for various purposes. The very best units have dynamic loudness control. The unit automatically shifts loudness curves with volume. The end result of this is that the music has exactly the same tonal balance regardless of volume which is very spooky when you first hear it.