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.

 @ 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).

E—Thanks for such comprehensive clarity; I generally concur. But my simple comment (that it's easier to phase-match subwoofers-to-main speakers when all speakers are sealed, not ported) was never intended to apply to idealized environs filled with bass traps and EQ correction. Nor did I intend that phase matching be conflated with time-of-arrival correction. It's generally not possible to accomplish the latter in the average home living room because decor dictates that the subs go into the room corners and the mains go up front, more centered. Given these typical limitations, it will always be easier to synchronize the phasing (at the xover frequency, at the prime listening location) if the speakers are sealed—not ported.

The extent of the phase convergence will be determined by the prevailing acoustics. In my own living room (no bass traps, strict WAF control), measurements indicate that closely matched phase won't prevail beyond a few feet from the prime listening location. Regardless, accurately phase-matching the subs/mains yielded significant audible improvement, and an instrumented means to accomplish the match was lots less tedious than doing tweak-and-listen trials.

prof—I'll try to help. It sounds as if you've got TWO separate subwoofers. Correct? What make/model are these subwoofers? And are these subs self-powered, with their own (independent) internal power amplifiers? If not self-powered, how do feed audio signals to them, and from what source?

prof—I certainly CAN help you with this; I own a pair of JL Audio E110 subs myself, and use a Marchand XM66 active crossover, which is functionally equivalent to JL Audio's CR-1, but lots cheaper (has lower quality internal components). 

However, I now have a dentist appointment to attend, and it involves some major stuff, so I probably won't be in shape to respond further today. I'll get back to you tomorrow for sure.

prof—OK, let's go: First, make certain that both of your e110 subs are free of a common component failure (bad or intermittent input level pot) that plagued many e110 plate amplifiers. Turn the input level gain all of the way down, and connect an input sine wave signal of 80 to 100 Hz at 1 Vrms to an input jack. Then turn on the e110 sub and slowly, smoothly advance the input gain pot to assure a steady increase in the acoustic output. If operation is normal, good. But if the output gain is not seamless—if it suddenly jumps to full gain—you have a JL Audio warranty repair claim to lodge.

( I purchased my own e110 subs in mid-2016. One plate amp input pot went bad a year later, and the other plate amp went bad in early 2018, This is a known issue to JL Audio. If your pair is affected insist on free warranty repair, and firmly insist of free pre-paid shipping. Do not return the entire e110; just ship them the plate amp. Disassembly is obvious.)

Let's get to setup:  (1) Placement: In most situations, the best acoustic results will be assured with the subs flanked outside the main speakers and near the room corners. That's also consistent with most decor options.  (2) Crossover Frequency: Pick your xover point based on the capability of your main speakers. If you're using small mini-monitor main speakers (like my own Spendor S3/5R2 units, with little 5.5 inch Ø mid/woofers), pick a high xover, I use 94Hz. If your main speakers are big floor-standers, use 80Hz. And don't go any higher than 96Hz, nor any lower than 80Hz.  (3) Set your (absolutely terrific!) CR-1 active external xover controls at the desired xover frequency (set subs/mains same), select full 4th order Linkwitz-Riley 24dB/octave operation, stereo operation, and set the CR-1 balance control at zero, straight-up. Set both of the CR-1 damping controls at ≈ +2.  (4) Set the e110 subs for (manual) power off, polarity switch at 0, phase angle set at 0˚, and select "bypass mode" operation, thus assuring that the internal low pass filters will be bypassed and out-of-circuit. All subsequent crossover control, except for initial setting of the e110's input gain and phase angle adjustment, will then be accomplished at the CR-1. This represents a major convenience advantage: It will allow you to readjust the subwoofer-to-main speaker acoustic output ratio from one centralized location—no need to crawl to each individual subwoofer.

(5) Verify that your preamp/main system volume control stereo outputs are fed to the CR-1 input jacks, and that the CR-1 high-pass stereo outputs go to your main speaker's power amplifier, with the CR-1 low-pass stereo outputs routed to the left & right channel e110 subwoofer input jacks. (Do not use a Y-connector at the inputs; just route the CR-1 low-pass left output to left sub's left input jack, and the right output to the right sub's right input jack.)

(6) Adjusting Subwoofer Input Gain & Phase-Angle: The desired objective is to…
(a) Set the subwoofer's input gain so that the acoustic output level of each subwoofer will be +3dB greater than the related main speaker's acoustic output at the specified xover frequency, at the prime listening position.
—and—
(b) Set the subwoofer's phase angle so that a xover frequency sine wave signal from the subwoofer is fully phase-synchronized with an equivalent signal from the related main speaker AT THE PRIME LISTENING POSITION.
Both of these adjustments can be readily accomplished in a direct, single step process. First, drive BOTH main speakers alone (subs off) to a high acoustic level (at xover frequency, SPL 82-to-84dB, C-weighted), as measured at the prime listening position, from a fixed sine wave signal source, using the main master volume control to adjust the output level. Then, set one sub (other sub off) for REVERSED POLARITY operation, and turn that sub on. Assure that the master volume control is at the same (loud) level as previously set. Now, ALTERNATELY adjust the subwoofer's input gain and phase angle controls to achieve a minimal null of the combined acoustic output, as determined at the prime listening position. This null will be very sudden, distinct, and pronounced. Alternately adjust both input gain and phase angle for minimal output. When done, switch off the subwoofer and return the polarity switch to its normal zero position.

Repeat the above procedure for the other subwoofer. When done, assure that both subs are properly reset for normal polarity output, and that their power switches have been reset for normal "auto-on" mode operation.

I have available a detailed two page white paper that describes the above process in detail. It also provides info on how to accomplish this setup with a basic dynamic microphone and the use of some simple instrumentation. This instrumented-means for handling this job provides helpful visual guidance, and eliminates the need for any assistance when conducting the testing. I can e-mail a pdf of this paper to you if you will provide me with your e-mail address. My own e-mail is: geyer.bryan@gmail.com

Advise if you have further questions.