We give up perspective to avoid tone controls

After 5+ decades of heavy DIY audio experience, I conclude that conventionally implemented (preamp) tone controls introduce too many inaccuracies to prove generally acceptable. Virtually all such circuits cause significant channel tracking imbalance, and some add other phase and distortion anomalies that seriously degrade any aural benefit.

In those cases (like mine) where the primary objective is to implement convenient variable control of the low bass-to-main speaker balance, there's a better/cleaner way: Add a pair of (sealed, not ported*) self-powered subwoofers. Operate them in "bypass mode" (internal low-pass filters deactivated), and control them through an EXTERNAL active crossover (Linkwitz-Riley, 4th order) control unit, e.g. Marchand's XM66. Fully variable control of the crossover point AND the main/bass input level is then available at a convenient single, central location. This makes it easy to set/reset your desired main speakers-to-subwoofers output ratio, using accurate plus/minus 1dB stepped attenuator front panel level control switches. It's an easy, elegant, and very accurate way to control the relative low bass-to-main speaker acoustic blend—and you can alter or restore a desired mix with optimum convenience.

*Sealed-subs can be more accurately phase-matched with your main speakers than when using ported subs. It's also helpful to use sealed (not ported) main speakers.

Virtually all such circuits cause significant channel tracking imbalance, and some add other phase and distortion anomalies that seriously degrade any aural benefit.

vtvmtodvm:
That's pretty breathtaking in it's scope. While I do agree: I wish manufacturers paid more attention to the quality of the tone controls, in general I have to disagree. I think the convenience factor needs to be weighed in. Like loudness controls for instance, being able to "convert" my speakers from medium to low-volume listening is a good thing, not to mention transforming them into party mode speakers.

Also, placement. Being able to adjust for speakers too close or too far from the walls is a case where tone controls are a lot cheaper than buying new speakers.

*Sealed-subs can be more accurately phase-matched with your main speakers than when using ported subs.

Since the ports contribute only to the bottom end, and the phase matching must occur at the top, I don't follow your logic.

There are many who feel sealed subs have the best transient responses though. In my experience, this is more a matter of integration and EQ than any absolute technical superiority of port vs. sealed.

It's also helpful to use sealed (not ported) main speakers.

This is often true. This is what THX tried to do with the satellite specifications, and has to do with phase and amplitude matching. They were big fans of LR4 alignments, and sealed sats + 2nd order HP electrical helps achieve this rather consistently.

One of my full-range listening systems is my home studio where I have access to 3 different hardware equalizers, each in the 3k-$5k price range, (no not graphic EQs).  They all sound great, and they each sound different from one another.  Each can make music sound better when it's lacking in some area.   I will use EQ when playing vinyl that's had the lows and/or highs rolled off to fit the format..... and make mediocre pressings sound far more entertaining.    So what's the alternative?... To sit there bored to tears with flat sound and my purist morals intact?  Life's too short for boring sound.  My bypass button works just fine for the great stuff.

I have this problem with my main speakers-not my subs and dot ant to lose my good sound, going thru my subs as a crossover. I have a Casablanca 3, (just taking up space) I guess I could put in the tape monitor loop?? It's very versatile. Guess I need to invest in a good parametric EQ. I'm considering building a new sound room, if I move. Don't have the bucks to do it the acoustic fields way. Think I'll start a new thread. 

 @ erik squires—Phase matching of subs-to-mains should always be optimized at the applicable crossover frequency, and the measurement point to define the match should be located (precisely) at the intended listening position. When ported speakers are involved, crossover frequency waveform propagation becomes more indistinct, diverse, and increasingly affected by room-induced peaks and nulls that muddy the matching precision. (I use instrumented-means to accurately accomplish phase matching. If interested, request my related white paper. I am at geyer.bryan@gmail.com)

Sustained wide area phase matching is a fantasy, and any expectation that synchronized phase can persist over more than a small part of the home listening room, or beyond the limits of a given test frequency, is misplaced. This reality should not be construed as a significant shortcoming; refer 4.8.1 of Floyd Toole’s epic “Sound Reproduction”, 3rd edition (Routledge, 2018, ISBN 978-1-138-92136-8).