Sound Stage and Imaging

Do you have more choices of crossover points? With one subwoofer you can not go as high as 100 Hz because it will allow you to localize the sub which is not good. You have to use at least two subs. But, you can probably take it as high as 80 Hz if the slope is 18 dB/oct or steeper. The other thing you do not want is a sub getting into your midrange. 

The less distortion the speaker creates the better the image. I think the stage is a more a matter of the recording, it most definitely changes with recordings from "in your face" to Symphony Orchestra wide, front row to back of the theater. The image is also recording dependent, but there is certainly a system element. Some systems can image others are more vague about it. The key is less distortion, phase aberrations and finally the frequency response curves of both channels need to be identical. This last part is not so easy to do. You have to be able to measure both channels (always a good thing to do) then you have to be able to modify those curves. Very few systems have the ability to do that effectively. Plain PEQ will not work. You have to be able to draw target curves which means more advanced digital signal processing.  

@riccitone 

You are absolutely right. I have set up systems with LS3 5As on stands with subwoofers and blind folded you would think you were listening to much larger speakers. They also image better than many larger speakers and there is less enclosure coloration. You have to run the subs up to 100 Hz to get the best effect which requires very steep cutoffs.

There is one issue I think you need to think about. The ability to move "fast" determines a driver's high frequency limit. Most subwoofer drivers run easily up to 500 even 1000 Hz. 100 Hz is never a problem. The larger the driver the slower (shorter excursion) it has to move to produce the same frequency at identical volumes which is why larger drivers have less distortion and frequently better transient response, just the opposite of what many people think. The same is true for multiple drivers as it is really a surface area thing. To make really accurate and powerful low bass IMHO you need at least two 15" or four 12" drivers in most average size rooms. I use eight 12" drivers. Above 15" the cone becomes more difficult to control. I have seen strobe films of 18" drivers moving in some wild ways, anything but pistonic. I would use Eight 15" drivers but the size of enclosure required would not work in my room. 

@mijostyn

Ive always wondered about using larger driver subs. Thank you for explaining those size limitations, good to know from someone that has actually experienced the effect of between 12 and 15inch drivers. Can only imagine how amazing your setup sounds!

I’ve only (currently) got a rel t/7x. I do like it a lot, but hoping to move to something open baffle, and two of them (each with a 12 or something to that effect) to better balance the room and control modes, blow open a more true stage. I’m sure it would become a bit of a crossover adjustment nightmare, and might need to get into some software but would be so cool. Am waiting until I might move into a larger space, however. But even if my current space is relatively small, wondering what at least a second rel sub could do.

I can passionately recommend a used pair of Alon (now Nola) 4 or 5's, which can be found for less that $2k. The open baffle mid and tops provide incredible soundstaging with pinpoint imagery when driven by a big solid state amp like a Krell ksa 100. The sealed 12 inch bass driver provides mind-boggling bass.

I keep a hot rodded pair of Alon iv's (they have the mid range driver and tweeter from the v's, also used in then range topper Phalanx, easily done with help from Carl at Nola) as my spare speaker. They were my main speakers for over ten years, loved them so much i couldn't part with them.

You didn't mention the genres you enjoy, but the Alons were a triumph whatever i played through them, from jazz soul and trip hop to deep house and drum and bass.

As mentioned, my single sub is 18". Don't want to trust it to handle 100 Hz (and the Nautilus are not known to be 'weak down low'.

But I have to go high in my small setup with various small bookshelf speakers, some of them do NOT feel good handling 80 or lower (certainly not at higher SPL), so there i need to drive the DD15 sub to 100 Hz.