Tone controls, treble, bass, and sometimes midrange, have been with us since the first full-featured tube table radio in the 1930's. But perhaps it's finally time to retire these features since tone controls are about useful (and precise) as a button marked "more."
What is a tone control? Basically, it's a volume boost at a pre-selected frequency point with a fixed Q. The pre-selected point is pretty easy to understand, but what is Q? It's a way to quantify the severity of the slope and range of a tone control. But what I've described also applies to a single bank of a multi-bank equalizer circuit, such as what you'll find on many audio devices, including iTunes.
Actually iTunes has a pretty sophisticated group of tone controls and pre-set EQ settings built into it. Also you can link tone settings to particular songs, so when you play them these settings will be automatically activated. In other words you can, if so inclined, have individual tone control settings for each and every song in your iTunes library.
If you use Pure Music as your primary playback software you can install multiple EQs in Pure Music's plug-in control pane. Amarra also has provisions for tone or EQ adjustment via three variable-point, variable Q equalization channels built into its interface. Tone controls can, and probably should, be handled in software, where not only the EQ parameters can be more precise, but also group delay and phase-shift issues caused by the EQ adjustments can also be addressed.
The best set of tone controls I've used on a preamp were on the Quad 44 preamp. Unlike almost every other set of tone controls, the Quad permitted changing the parameters of the EQ curve, especially on the bass. In comparison your average tone controls, with their permanent frequency points and fixed Q are very much a hit-or-miss proposition.
Many receivers and Pre/pros now include sophisticated room-correction and EQ features that make tone controls seem as quaint as 45-RPM record adapter. Unless the tone controls are adjustable both in frequency and Q, manufacturers would be better off leaving them out.