Please excuse the essay, but I must beg to differ with the eminent Kal here, and also the extension from his comments that Macrojack suggests. Unless you're very careful, patient, and schooled, you can do a LOT more harm with an equalizer than you can with well designed tone controls (bass and treble). I understand why 'audiophile' preamps historically began eschewing tone controls (they didn't always -- some founding marques of the high end preamp business used to have them back in the day). The preamps I've owned haven't had any in years, and I realize they may not be as needed today as they once were, from more than one standpoint. But judging from this thread and others, and the audiophile preamp marketplace, I think tone controls are unfairly denigrated and undervalued these days, largely as a result of marketing propaganda and received wisdom that no longer need hold true, if indeed it ever had to.
Macrojack says they only "affect one frequency", but this isn't how 'shelving' tone controls work. It's true that a simple tone control might only have one, non-selectable 'turnover' frequency where it begins its effect (below that frequency for a bass control, above it for treble), but the control progressively increases or attenuates all the frequencies beyond that point, usually at least a couple of octaves' worth of range. Another way of putting it is that tone controls are 'low-Q', i.e., don't have a peaky, highly resonant nature that focuses their effect on a narrow slice of frequencies; for that kind of very specific correction, you do need an EQ. (I own but seldom use one of the best ever offered for home stereo, the 3-band full parametric from Sony's late, lamented Esprit division, of which extremely few were made. Maybe only the Cello was better.)
But a broad yet subtle action is sometimes exactly what's needed to help compensate for the vagaries of mastering and recording quality. We're not talking about room correction here -- that's a separate and much more involved issue. With real-world recordings that can often show either a treble that's slightly too dim or bright, or a bass that's slightly too lean or heavy, it's quite possible to get a genuinely improved result (i.e., higher in fidelity to the original event) through the use of simple but intelligently designed tone controls. Incremental, 1-3dB adjustments covering the high and/or low ranges of the frequency spectrum can sometimes make a world of difference between sound that's noticeably lacking or overbearing and something you can more easily 'relax into', or at least tolerate. (Of course, for those tragic audiophiles who've traded sonic perfection for musical corruption and simply banished any recordings they ever loved which exhibited such flaws, you may be excused, without my sympathies.)
The key -- other than using high quality filter circuitry and providing some sort of defeat option (assuming this is being done in the analog domain) -- is to keep the frequency regions affected mostly out of the broad midrange, and to use gentle contour slopes offering a moderate amount of boost or cut that comes in gradually. NAD for example has done this successfully for years, and although their products that feature tone controls aren't what audiophiles call high end, anyone who's used this type of well-engineered control can attest to its helpfulness in aiding the playback of less than perfect recordings. MacIntosh is a higher-cache, highly historical brand that's also never seen the 'wisdom' in doing away with tone controls entirely.
Part of what I think gave tone controls a bad rap among audio enthusiasts -- other than a general desire for keeping circuitry as simple as possible and a largely misapplied popular phobia of phase distortions -- were mass market receivers and preamps with tone controls that increasingly went the other way, purposely designed to heavily encroach on the presence and warmth regions of the midrange, with steeper-slope actions that provided for excessive, discontinuous boost, to enable the kind of 'boom and sizzle' no-fi presentation which tends to impress the young and the innocent and diverts attention from things like fundamentally poor speakers. (Somewhere in there also lies the notion that, despite all the distortions -- both random and introduced -- which inevitably must intrude on our recording and playback capabilities, as good audiophiles we can always smile and point to the false fact that, hey, at least we don't do anything 'deliberate' to adulterate the 'purity' or 'accuracy' of the signal. Ha!)
But it's perfectly possible for a high end manufacturer to put as much care and quality into a tone control as they would a volume control or an RIAA or crossover network, using the best parts and judicious design. Such a conservative implementation isn't as amenable to abuse, and anyway knowledgeable audiophiles presumably wouldn't try or want to use their tone controls that way. It would add cost, but if you look at the expense of the cable and tube games and so forth audiophiles love to play, often in a pursuit to banish the everpresent threats of 'brightness' or 'edge' or 'leaness' on their favorite non-audiophile recordings, and the prices top preamps fetch today regardless, this doesn't seem to be the culprit.
The real problem is that the die has long been cast, and audiophile preamps for a few generations now have been identifiable in part by their relative lack of controls of any sort other than volume and usually only a bare minimum of I/O and switching facilities. Thus, audiophiles have come to 'know' that tone controls are bad -- they must be, otherwise we wouldn't be shunning them -- and so almost no preamps with high end aspirations can be introduced with tone controls in this day and age, because to do so would probably mean marketplace suicide no matter how benign and effective the implementation. But I don't think anyone could seriously continue to make the argument that a high quality, defeatable tone control would really do more harm than good under any and all circumstances.
(Occasionally, especially since they went extinct in preamps, rough tone controls of a sort -- the analogy isn't perfect and the purpose not exactly the same -- have reappeared in audiophile speakers through the provision of modest boost or cut switches, usually affecting only individual driver ranges. But even this type of adjustability, useful as it may be, hasn't been too popular overall. This selectable filter-abhorance disease even seems to infect the phono renaissance -- despite the necessity of, at minimum, RIAA compensation, and the ever-expanding plethora of new offerings in a crowded marketplace -- exceedingly few preamplifiers seem to offer switchable rumble filters, whereas during the true era of the LP many if not most did.)
Perhaps ironically, maybe one thing that could start to change this state of affairs a bit will be the need for traditional 'analog' preamps to compete with a new wave of digital preamps that already do and increasingly will offer the option of not only advanced room and speaker compensation EQ, but also a digital version of essentially the same kind of bass and treble tone controls, for quick'n'simple 'touch-up' use on a recording-by-recording basis, that we abandoned in our heyday. (Nahhh, I don't really believe that -- the purist analog preamps will probably further solidify their bare-bones approach, to wear as a badge of honor in the face of increasing digitalization.) In reality, the supreme irony is possibly that, as decadent audiophiles, we are often at odds with the 'slide-rule objectivist' crowd as to just what it is that audio reproduction ought to strive for. "They" say accuracy, as defined by measurements. "We" frequently say, that's great, but not at the sacrifice of personal enjoyment and aesthetic emotional involvement. "They" say the newer technology is demonstrably superior. "We" often feel, OK, maybe it is in certain ways, but let's not forget about some stuff they seemed to get more right in the past. Now, apply that paradigm to which camp it is that vehemently rejects old-fashioned, subjective tone controls, and tell me what end is up?...
