I don't have any fundamental objections to "tone controls" in Audio Playback Systems. I also don't have any objection to purist systems that don't have them.
Now, Graphic Equalizers, that is a different kind of animal. Again, no fundamental objection, but being familiar with many of the kind, most simply don't sound that good from the start. Trust me, when you install one into a Mid-Fi nationally advertised system and you can hear whether it's in or out when set to 0dB correction ... that's not a quality audio product.
Of those that could be considered "neutral sounding", say, a Pro parametric or 31-band graphic ... well they aren't actually that neutral sounding. You can often tell whether they are in or out of the playback chain, even when set to no equalization.
The DSP-based units are better, but not better to the point where the effect is inaudible.
So ... what to do? Tone controls are there to mitigate the issues that arise when you take Speaker A and install in in Room A. Take the same speaker and install it in Room B and it will interact with the environment differently, and so there is your need and your solution.
Graphic and Parametric EQ are different than something like a Bass cut / boost knob. Essentially, again, I have no fundamental objection ... in other words as a concept I understand the attraction ... but as someone who spent many years configuring various systems for retail customers, I cringe at the idea.
The problem? Virtually in every case, the adjustments are just too tempting to ignore, and users just mess up the sonics when they put their hands to them. So, a system carefully assembled to provide a synergy (if you don't do this, you don't belong in retail or consulting audio) is then changed to a messy shadow of it's true potential.
Once you get to a certain point ... and it's not based on how much you spend, but on how well the system was configured ... you should be dealing with these issues without resorting to tone controls. If judicious use of tone controls are employed ... I have a fondness for frequency tilt controls like those found on QUAD preamps and Luxman's Linear Equalizer* ... no problem. But that is so rarely the case, and once you leave the home after setup, they are so often altered by the owner, that I can't really advocate their use.
Maybe I want too much control and I should just back off and let people do what they want with what they bought. But it's frustrating when a solid, excellent sounding system in a given environment is degraded by users. Sorry, I know that is somewhat elitist, but it's how I feel about it.
When it comes to just the LF region, say below 300 Hz, things are a little different. You can't just lengthen a room wall 15 feet when the need arises. But LF is also much easier to do circuit wise, so transparency and other sonic signatures of equalization are much more difficult to detect. Digital is almost perfect at LF ... that's not the end of the spectrum where digital has it's issues. But above that, I'm not convinced anything beyond subtle adjustments have any place in a domestic Sound System.
* These controls alter the spectral balance of a system in small but meaningful ways. For example the Luxman Linear Equalizer knob, which actuates a switch with usually 9 total positions in +/- 0.5dB steps, turned full counter-clockwise, would increase the LF and simultaneously reduce the HF, for perhaps a +2/-2dB adjustment. If you were to imagine the frequency response chart, it would have a down-tilt from 20 to 20KHz versus a flat line with the control out of the circuit.
If a difference of 4dB at 20 vs. 20KHz isn't enough, your system is clearly improperly matched, and the problems should be addressed elsewhere. In fact anything beyond about a total change of 2dB is probably in indication the system needs attention.
