Are big subwoofers viable for 2 channel music?

Hello smodtactical.

     There are many advantages to using 4 subs in what's called a distributed bass array (DBA) system.  Using 4 subs, that are each rated to have a capacity of outputting bass down to 20 Hz, results in no single sub being required to operate anywhere near its limit, the cumulative bass produced will effortlessly and accurately reproduce whatever the music or HT source content demands down to the deepest audible bass frequency of 20 Hz and the bass will be fast, smooth and detailed enough to seamlessly integrate with any pair of main speakers you choose to utilize. This high quality bass will also exist throughout your entire large room and not just at a single designated listening seat.
     You have the option of either creating a custom 4-sub DBA system with 4 subs of your choice or buying a complete 4-sub DBA system kit such as the Audio Kinesis Swarm system for $3K reviewed below by the Absolute Sound:
https://www.theabsolutesound.com/articles/audiokinesis-swarm-subwoofer-system/

     Due to the large size of your future room and your desire for high quality deep bass performance, I suggest you utilize a minimum of 2 good quality subs, if you're mainly concerned with high quality bass performance at a single designated listening position and 4 good quality subs if you prefer even higher quality bass performance throughout your entire room.

Best wishes,
     Tim

Tim would you go for 4x rel 812, swarm or dual rs2 / s7012?

OP must have missed the very first post. Tim and I are on the same page. In fact we have almost identical subs. https://systems.audiogon.com/systems/8367 Main difference is I built mine, with help from him and Duke, while he bought Duke's Swarm. Also I have an additional 5th sub, the Talon Roc. Which is how I know the more the better.

Like the first post said, its not a question of which, its how many, and the more the better. 

Subs in a system are like horsepower in a car- more is always better.

" Tim would you go for 4x rel 812, swarm or dual rs2 / s7012?"

Hello smodtactical,

     I'd recommend the Swarm because:

1.  It'll be simpler to setup since you'll only need to properly set the volume, cutoff frequency and phase controls once on the included 1K watt class AB amp/control unit for all 4 subs, rather than individually for each sub with the other options.
2.   It'll be much more aesthetically pleasing in your room than the other options.  Each sub is relatively small at 12"wx14.5"dx28"h, is supported by 3 conical spiked metal footers, have the speaker wire terminals hidden by being located on the bottom and are positioned with the 10" driver facing, and about 1-2 inches away from, the wall.  Once properly positioned, the subs look like attractive wooden pedestals in the wood of your choice.  You can also utilize them as small end tables if you'd like.
3.  It would probably be the least expensive option, unless you know of a seller who offers large discounts on REL subs.

     Overall, four subs will also perform about twice as well as two subs in terms of bass power, speed, smoothness, dynamic impact as well as the levels of detail and seamless integration with your main speakers.  I'm not certain but believe the 4x REL 812 option has the potential to marginally outperform the Swarm but would be more expensive and require the setting of the volume, cutoff frequency and phase controls individually for each sub.

Tim 

I have a pair of REL 212s, main speakers with their own powered subwoofers. So far the best sub position has been to place them just a bit beside the listening position, slightly to the rear. Without one of the various room measurement programs everything seem like a crapshoot. 
Not to say they haven't improved the bass and overall presentation, but there remains questions as to whether they are optimized.