A pair of REL’s s/510 would provide seamless integration through high level connectivity and superlative bass for 2 ch & HT.
Brace yourself...subwoofer discussions here ends with no consensus!
The a crossover with high pass is the most important part. With a good crossover you can greatly improve your sound. With just a low pass you are just extending the bass (which is fine too...but not as good). Two subs will be much better. One sub can work fine for a single seat but it is much harder to get right. I would take two cheaper subs over one pricy sub every time.
I can’t comment on the subs you are looking at. I have never really demoed subs back to back but l like sealed high powered subs for music.
Interestingly I have found that all most all the speakers I have tried crossover best right around 60hz. Something about 60hz seems to optimize both the sub and speakers. Maybe not forcing the sub to play too high or the mains to play too low. Not sure but this has been with multiple speakers and rooms. Anyway give 60hz with a high pass a try.
Also keep in mind that speakers will cancel out the bass 1/4th the wavelength to the rear wall. If you put your speakers 4ft off the wall there will most likely be a hull at 50-55hz. You can place the sub less than 36” (from the driver face) from the wall to avoid this cancellation. Another options is bring everything more than 6’ off the wall to avoid bass cancelation but this is not practical for most.
Here is a chart that can help. It will be quicker to calculate it for a starting point. Between 3’ and 6’ the bass with have major issues but this is where speakers should go for soundstage and midrange tone. But placing the subs (less and 36”) and speakers in different spots can fix it.
This video might help too. Not exactly related but will make more sense than my rambling lol.
The most important thing with subs is to have a lot of them in different locations. Four small ones distributed around the room is way better than any one or two no matter what or where or how powerful. This is a lot easier to say than to understand so if you are serious then do a search and read up, you will find nothing but screaming praise from those of us who have actually done it all saying the same thing. Also those who have not (and therefore do not know) are not shy in spreading what they do not know. So you kind of have to read past that. Try.
If you can really only have floor space for one, or two, then Tekton 4-10 is the way to go. Get two.
And just to be clear this is not to have two times or four times as much bass. The reason, which you will find clearly explained all over the place if you look, is every sub no matter what or where you put it generates peaks and nulls. Using more subs in different locations creates more peaks and nulls in more places. But having four lets you run each one at a lower level. The result is they all sum together into much smoother, deeper and more powerful bass than you can get any other way.
There's no such thing as "overkill", but at the same time, your goal shouldn't be to "pressurize" the room (unless you're listening to Bassnectar at 11), but to seamlessly blend them with your speakers so that they don't call attention to themselves. You want them to deliver all the bass that's there in the music, the lower frequencies that most speakers can't deliver, but not overpower the midrange and treble.
I think you get a lot of bang for your buck with Rythmik and I like their servo technology, but also like SVS and Power Sound Audio. Get as much and as many subs as you can afford. Spreading out the load and using multiple subs to cancel out peaks and nulls is a good thing. My room is a little bigger than yours and opens into a kitchen/dining room area. I have two Rythmik F25s and two SVS SB13 Ultras (both are sealed subs). Not overkill.
In my system I am currently using a REL S5 SHO, a Rythmik F12SE and two SVS SB4000’s. My room is 10 x 14. When I first started using the REL, I used the speaker level input but have since changed to using the rca inputs. To me using the line level inputs there was slightly less distortion giving me a slightly cleaner sound. The Rythmik has the best sound of all three though it has a little less output power. The SVS has the most output power of the three, but the sound quality is not quite as good as the Rythmik but a little better that the REL. The sound quality differences between the three are noticeable but not substantial. If you are not using some type of external crossover the Rythmik and SVS also give you more tools to help with integration.
If multiple subs overwhelm a room that is just an integration error and can be easily fixed. The intent of multiple subs is not to create more bass, it is to balance out the bass through out the room. It is a type of acoustical treatment for the room as explained by MC and Big Greg.