Eh hem!...Subwoofers... What do ya know?

n without using high pass filtering, matching the stand alone subs to the speakers presentation is impossible.

@m-db is not impossible, on my specific case the subs and the speakers are manufactured by the same company, speakers crossover and speakers themselves are tuned to the 4 subs and integrated with dsp amps.
The beauty of it is you can follow the designers integration recommendations or you can try to play and better the designers intentions (not that you could have a real chance to improve it much) tweaking dsp etc.
As and added convenience 2 out of the 4 subs share the same footprint as the speakers so no extra space needed. I have to add that I personally like the idea of separate subs from the speakers better in terms of location, true you need some extra real estate but not much and you have more options to locate the subs, you can even tweak with the vertical plane and locate 1 or 2  in your ceiling if you are up to the task (I am not)
In the case the designer is not the same you still can find speakers with proper integration, most speakers are ranged starting at 50 hz 60hz you could use dsp to match these values as well or modify the xover like you mentioned

Erik said it best, " Hard to integrate well. Glorious when done right."

I've used subs for many years now, mostly to augment stand mount two way's.  Up until I had my current setup, my system had to work the best it could in our living room, with minimal options to make changes to the room to accommodate the sub.

Now that I have a dedicated room, one that is much larger than the living room and whose dimensions are nearly square, integrating my current subs was incredibly frustrating!  Before applying room treatments, the only way to minimize the room adding a single note to whatever the bass line in the music was, was to position them behind the sofa, in  approximately the mid-point of the room.  The problem with that was the "disconnect", where the performers seemed to be in front of the listener, but the impact of the low bass came from behind.

I eventually was able to "tame" the added "single note" by purchasing a calibrated microphone and REW, to help place 10 bass traps.  Now when I listen, as my wife says, it's in my "padded cell".

pretty solid article on sub integration.
https://jlaudio.zendesk.com/hc/en-us/articles/205061040-Adding-a-Home-Audio-Subwoofer
I am preparing to integrate a pair of JL E112 subs in my system...showing up in a few days...
I will first try high level connection right from the amp terminals to the subs HLE input...then try a Jensen Iso-Max speaker level to line level isolation device...I think ultimately though the JL CR-1 (or similar) the ideal solution for integration mostly because the latency can be better controlled;

I initially plan on placing them like this (on the insides of the speakers);
https://imgur.com/Paj46Ti

feedback welcome!

Speaker level inputs are subject to the amp/speaker impedance matching. 


Since I've heard a number of examples where a dropping impedance in the 100Hz region causes dimished impact, this would not be my first choice. 

+1 for measurements

+1 for Mini-DSP

Audiophiles make fun of the home theater crowd. The home theater crowd makes fun of audiophiles. Both groups can learn something from the other.

Subwoofers should be sized according to the room. I had Vandersteen 2ci speakers in a small room approximately 11x14 and they sounded great, I moved them to a much larger space and the magic disappeared. I did not need a subwoofer for the smaller room. I did need a subwoofer or more for the larger room.

Timing matters! I do not have automatic room correction. I use two JTR, S2 subwoofers in the basement. A friend familiar with free REW software came over to help dial them in. My room is 29 feet long. One subwoofer is in the front left corner. The other sub is located in the right rear of the room. The subs were level matched so they both played at the same volume at the major listening position. The subs were then individually measured and the individual measurements laid over one another via the program. The goal was to make two sign waves from two subs appear as one. The delays were made in the Mini-DSP until the sign waves were as close as possible.

My friend then proceeded to time align each speaker to the subs. Again the goal is to over-lay all individual measurements and have them appear as one sign wave. We did not get it exact. We gave my multi-channel system a tune up. Now instead of an 8 cylinder engine running on 6 cylinders I had a system working together as intended.

Prior to my friend coming over, I had pulled out a tape measure and set speaker distances. The distance measured by tape was different than the distance measured electronically. After timing alignment, I was able to reduce my listening volume by 10db and maintain the same enjoyment and listening intelligibility. I know many use automatic room correction and assume it is correct, you don’t know until you measure. You cannot just do the math to determine delays, the room and things in the room affect how sound travels. According to the math I should need 7-8 seconds of delay. Measuring with a mic resulted in 5.811 second delay.    

After alignment my system measured +/- 3 Db from 100Hz to 6Hz. I tried to listen to my flat system and did not enjoy it. I tried increasing the overall sub volume, but that resulted in bloated bass. I finally did some equalization in the Mini-DSP and the system came alive. I was able to reduce the overall sub volume back to where it initially measured flat. As someone mentioned earlier, we do not hear all frequencies at the same level. Lower frequencies must be louder to be heard evenly. Equalization helps accomplish this according to individual taste.  

My system produces great bass for movies but for music you may not know the subs are playing. After the changes, there have been a number of times I have really been surprised at the bass content while viewing non-action movies.

I use Innersound Eros speakers for two channel listening. The speaker is essentially two electrostatic panels on top of two ten inch transmission line subs. Bass is omnidirectional but there are recordings where two instruments that produce notes below 80 Hz are used. There are recordings where two pianos or two drum sets are used. A properly set up system can reveal that multiple pianos, drums, guitars are being used, just as a quality set of stereo speakers can reveal multiple singers harmonizing as one. In my humble opinion stereo subwoofers can help a listener detect the dual instruments in those recordings.  

           

In my humble opinion, from my humble experience