We give up perspective to avoid tone controls

I use 2 unobtrusive beautiful sounding REL subs (inexpensive bought used) and their volume pots are accessible due to chicken head knobs, and until recently they were my only "tone controls"....and that was fine, and mostly still is. I have a Schiit Loki that is also unobtrusive and seems to have zero impact when bypassed (click it on and off and you can see what that's all about...no sign of it). Other than the fact that, in my case anyway, it makes for longer cables runs to the amp (and is single ended only). I checked very carefully to see if the longer cable runs (a couple of meters instead of previously one) had any negative effect on my preamp-to-amp signal and was happy to notice it doesn't. In any case, those who are anti "DSP electronic nanny" (they remove some of the soul from music, and I should know as I have no soul at all) should order a Loki immediately. Although it stays bypassed most of the time, it's an extremely useful little el-cheapo gem. Look for a balanced version eventually, although that should be somewhat more costly...still...

vtvmtodvm,

Just noticed your reply.

I have two JL Audio E110 subwoofers, and the JL Audio CR-1 outboard crossover, as well as a DSpeaker Anti-node sitting in a box if I want to use it.

I have tube preamp, tube monoblocks, and intend to use the crossover to send signal to the mains and sub.   If I feel the need, I'll use the anti-node on the sub frequencies (I don't care to digitize the entire signal if I can help it).

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.

Brevity warning: Buy a sub, adjust the phase if possible so it works properly, move it around until it sounds good and turn it up or down to blend with the main speakers. Done.