Is It Fair To Fix It?
It used to be that "Mid-Fi" audio gear could instantly be spotted by its abundance of "features" (knobs, dials, lights, controls, and functions, for whatever purpose) and that the cheaper it was, the more of such toys and goodies it was likely to have. "Serious" (high-end) audio gear tended to be exactly the opposite the more you paid for it, the less it was likely to do.
The best of high-end audio electronics often had no "bells and whistles" at all and, though usually offers very high performance, tended to be simple to the point of austerity. Tone controls were, of course, out of the question. So was any form of Fletcher-Munson "loudness" compensation, or, for phono use, such mid-fi commonplaces as "scratch" or "rumble" filtration.
And a really expensive high-end preamp (there were no high-end integrated amps or receivers back then, so the preamp was about the only thing that might have "features") might not even have a "balance" control, but would provide, instead, separate volume controls, often concentric, for the two stereo channels. On a high-end preamp's ascetically bare faceplate there might be nothing more than a selector switch, provision for volume control, and single tiny pilot light.
The reason for all this was to maintain "sonic purity" by doing as little as possible to modify, distort, or change the recorded signal in any way, and thereby ensuring the greatest possible fidelity to the original musical performance.
High fidelity to the music was what the term "Hi-Fi" originally derived from and we audiophiles were quite serious in our goal of achieving it. That was why we (even before the appearance of the term "High End") allowed no tone controls or anything else that might change what our equipment sounded like, even if that change might have made an obvious improvement. The reason, of course, was that any improvement we might make other than by actually getting new and better equipment or by modifying what we already had (with new caps, deluxe resistors, changed internal wiring, etc.) was seen as "not playing fair" somehow violating the spirit of our "quest" by using cheap tricks instead of "honorable" (and expensive) new (or newly modified) gear.
We believed then, as a great many audiophiles still do today, that a really good system the system we all aspired to own someday should not need any help, either from tone controls or from anything else, (including "surround sound" or other electronic or acoustic tricks), in order to make a good and realistic sound. That's why, if our system's bass was weak or missing its bottom octave, or its treble sounded like it was playing through a mountain of pillows, or it is imaging or soundscaping front left too much to be desired, we would rather buy better or newer gear a subwoofer, perhaps, or new tweeters, or new whatever might be needed than simply to turn a tone control up or down, or to use some other kind of (easy but somehow not "honorable") knob, dial, or other "artificial" correction to make things sound better.
That's all well and good as far as it goes, and remains true, even today. Nevertheless, it seems that somewhere, deep in the heart of many of even the most passionately-Purist audiophiles there lurks a guy who, though he knows without a doubt that the very best the very purest and very highest fidelity recordings are made in only two-channels, either direct-to-disc or direct-to-tape; only in pure analog, using only two microphones, with no equalization or "echo" at all, and no post-production "mastering" whatsoever, still, even despite all that, yearns for something different. Perhaps he once saw a picture of a studio mixing console with a nearly infinite number of channels, inputs, equalizers, "phasers", "flangers", special effects, knobs, dials, slide-pots, and other instruments of infinite control and has lusted after it ever since.
As an example, consider this: Decades ago, buyers of the then probably-most-expensive-speaker-system... in-the-world, the Wilson WAMM, found nothing at all wrong with the fact that along with each pair of speakers came a Crown professional equalizer (a glorified kind of tone control) that David Wilson, himself, would use to tune the speakers to the buyer's own specific listening room. It was all part of the service. and nobody saw it as "unfair" at all.
David Wilson did it manually, back then and, if the policy he established about personally installing their top-of-the-line speakers still holds true, someone from the company may still do it now. Since that time, though, any number of computer-based automatic devices have come to market that may do the job even better, and may, in addition to just frequency-response, correct for other things (room reflections, group delay, etc.) as well.
The process just mentioned is known as Digital Signal Processing ("DSP"), and devices and programs to effect it are available in a broad range of hardware and software formats, at prices ranging from cheap or free to many thousands of dollars.
DSP is not the only kind of system-diddling that modern audiophiles seem not to consider unfair or "impure." One of the most common others, at least among fans of vacuum tube electronics, is "tube rolling" the practice of selectively changing brands of (usually) the same tube in a piece of audio gear in order to obtain some desired sonic outcome.
As I learned, probably in 1990, when I reviewed a pair of tube preamps for another magazine, changing tubes (in that case, from Chinese to Russian 12AX7s) can make a colossal difference in everything from the imaging and soundscaping, to the detail, to the overall bright/dark coloration of a piece of gear, and by proper selection, a skillful or lucky audiophile can make his system sound within limits, of course just about any way he wants it to.
The same thing can also be done, to nearly the same degree by (Yeah, Trolls, get ready to go for it!) changing cables; by changing from one kind, brand or model of electronics or phono cartridge to another; and, HUGELY, by changing speakers or even speaker placement!. All of those things apparently like DSP are considered to be "corrections", "updates" or "mods" (short for "modifications") and not to be "unfair", even by the most pure of Purists. Even extensive acoustic treatment and "real" (as opposed to DSP) room modification is, for High-End audiophiles (perhaps because it's not actually a part of the system), considered just to be another form of "correction".
However it's done, I think the old saying that "All's fair in love and war" applies to Hi-Fi, too, and that any kind of sonic improvement at all is just fine if it will help you to...