Sound Stage and Imaging

@mijostyn Yes, I run one large subwoofer crossing at 50 Hz, and run the Nautilus 804 as 'small' to relieve from 50 and below duty (also relieving the receiver from the heavy lifting for that frequency range). So the Nautilus woofers only have to handle 50-350. I have found that very beneficial also with other speakers/receivers combinations after initially running the the speakers/receiver as 'full range'  only 'adding' the subwoofer for the low frequencies. 

Smaller speakers that can handle power can throw some insane stage properties. Even in difficult spaces - to a degree. I’m floored by how my Spendor 4/5 Classics can maintain composure, naturalness, pump dynamics and throw deep and wide with care in placement (got them used for $1300). But having a good amp and pre at about the same price really made this happen too. What kind of amplification have you got going?

Tech is getting crazy good with powerful class D and chip amps if you wanted to get something affordable that can get you in the ballpark.

No need to go crazy in your room, but you will at least need to get some diffusion/absorbtion on your walls with symmetrical placements. Many diagrams out there to get you started. Not the only solution, but GIK can help with suggestions. Just send them pics and they’ll give you some free advice.

A totally different direction would be to save up and get a pair of active speakers with room correction capabilities via house software. Buchardt Audio has a couple of amazing setups with their own dedicated stands. A local dealer has a pair of small Cabasse Rialto speakers that would just blow your mind. Crazy good sound stage, and surprisingly refined tonality. Has its own dacs, streaming capabilities. Something like 1,000 watts total amplification separates between a concentric tweeter/midbass, and a powerful rear-firing sub. It has built in mics to aid with a cloud based room correction to boot.
But they’re $3.9k.

Any way you slice it, placement and at least minor treatment within the room is pretty essential.

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.