[roymail]

I've always wondered about this, so I will ask.

When tone controls are used, do they actually add or take away from what is what is recorded on the source.  That doesn't seem possible, and as I write this I'm feeling kind of dumb... 

So tone controls must be for those who want to change the sound that is actually recorded.  Why would anyone want to do that?  Maybe it's because the amp or receiver can't produce the proper amount of resolution or transparency that is actually part of the source recording.  Just a guess.

I really don't know, I'm just asking.  OK, what do you think... this may be interesting after all. 

[wywires]

Often useful with poor recordings. Often rock and pop recordings are way too bright and having the ability to cut the treble makes these recordings somewhat listenable.

[Æ]

Tone controls are kind of like spices/condiments. Some foods (music) could use them, some foods (music) don't need them. It's all a matter of taste, personal preference. Some people put Tabasco on everything, other people scrupulously avoid salt.

[decal]

They do not change the recording, just the way the equipment reproduces it. Now whether this is good or bad is strictly up to the individual.

[JLM]

Tone controls boost or decrease the gain to bass/treble frequencies.  The amount starts at a specified frequency and the boost/decrease is at a specified maximum rate, measured in dB's per octave.

Equalizers (digital, software based, analog) can do dozens of boost/decrease at different frequencies to provide more control.

[roymail]

These answers are all helpful and appreciated. 

[tesseract]

Tone controls (EQ) are used to fix problems, and the problem is most often the room/speaker interface.

Analog tone controls can also introduce problems, such as phase anomalies, and most EQ, digital or analog, generally balances the frequency response in only one listening position.
 
If the speaker/room interface is the culprit, best to address the root cause and fix the problem, physically. Move the speakers and/or listening position. Any EQ then, if needed, should be minimal.

[midfi]

Often useful with poor recordings. Often rock and pop recordings are way too bright and having the ability to cut the treble makes these recordings somewhat listenable.

yup yup!  I know exactly what you mean.  My pre has no tone controls (Adcom GFP-750) but I have a DBX 2015 eq.  It's rarely used but there are some recordings that without it the sound is unbearable and becomes fatiguing rather quickly.  This is a problem more so on Digital music, much less on analog stuff.  It's easier for me to move a slide here and there every now and then for a crap recording than it is treating my listening area.  I'm just sayin...

[redbook]

   Yes ,the equalizer is certainly an idea I have considered for years. My room and speakers are well balanced but nothing can be done for those terrible pop cds from the eighties except to alter the playback. I would use controls only as last resort and ,if so, the best would be the dbx or any other studio quality unit. (IMO).

[srb]

Because the ear is less responsive to both low and high frequencies at lower volume levels, tone controls can be helpful when listening at low volumes to correct for this deficiency, much like the fixed loudness switch or variable loudness control (as some McIntosh preamplifiers had) of the past.
 
The tone controls offer a little better control than the fixed loudness switch.  As long as the tone controls are able to be bypassed with a minimum of wire length routing, I don't see a problem, although purists would argue the downfall of the signal going through extra wire or trace length as well as going through a switch or relay (although they don't seem to have a problem with preamps that route the signal through other switches, like a phase reversal switch).
 
At normal volumes I agree tone controls should not be needed, but when I listen to music at very low levels, I appreciate being able to boost the bass and treble a bit.
 
Steve