Adding a sub. - can i start with one?

That said the only downside of having only one sub, especially if you want the sub to add audibly to the bass, is that the imaging may distract you as the low bass might always seem to be coming from wherever the sun is despite the bass player actually being on the left or right side, so be prepared for that. 

Absolute BS. As long as you are using a subwoofer as a subwoofer and not a woofer, there will be no localization of bass instruments. None.

Starting with one is fine. However, the issues of damaging the soundscape from a large subwoofer are typically due to running the sub too loud, using too high a crossover frequency, or too shallow a crossover slope. Likewise, localizing sound from the sub indicates it is outputting significant energy above ~200Hz. Even a 50Hz lowpass crossover at 6dB/octave will only be down 12dB at 200Hz and quite audiblle

@daveinpa
Good call on REL S/510. Once you get the second one added, you be glad that you went with REL. And be sure to use high level connection with REL, other subwoofer manufacturers are trying to emulate this but it’s not even close in terms of seamless integration with your main speakers and not to mention outstanding build quality.

@ozzy62 Whoa whoa whoa. That’s the reason I was asking what speakers the OP has, and what subs they are considering. The Audio Physic Tempo 25 goes down to 32hz. The Rel 510 goes down to 20hz, but at -6. I believe it’s more like 25hz or greater at -3db. The OP mentions missing some oomph and giving it some “kick in the sound”. That is desire for a very audible range, and it will effect the soundstage perception. Also, subs do more than just add bass, especially if they go low enough below 20hz - they clean up the rest of the soundstage and bring clarity to the rest of the frequency response. And yes, it is possible that one sub in a corner will bring more of that clarity on that side… affecting the imaging. This is why many people prefer two subs or more, for an even response through the room. 

@daveinpa To build on my last post above, you will need to decide on what you want the sub to do. Given you have over 700sqft (that’s over 20’x30’!) and really high vaulted ceilings, if you’re wanting guttural impact, the Rel 510 may not do it. This is not to say Rel is bad, but with their frequency response cutoff, they are designed for smaller spaces than yours and will mostly add only audible bass on the low frequencies.

The one other big benefit of subs is as I mention above. With proper integration and the ability to generate lower moving waves, subs can improve phasing issues and bring openness and clarity through the frequency spectrum and improve imaging such that everything “snaps in place”. For that to happen in your room, I believe you either need a sub that can go lower and is still “fast”, or you need a swarm like mentioned above.

Again, this not to say that one sub is not bad, but to reach your goal, you may end up choosing a different sub or path. I learned this the hard way and had to upgrade my subs almost immediately.

I love Rel, I really do. But they have limitations in larger spaces until you get to the 812 or higher, or unless you get many of them. For less, I really like the SVS SB4000 or 16 Ultra sealed subs, and if you play your cards right, you could get a pair used for $3K shipped. The SB4000 goes down to 19Hz at +/-3db, and you can integrate the sub on the fly from a mobile app in your listening chair… everything from volume, phase, crossover frequency, and parametric EQ. I’ve found being able to adjust phase by degree from the listening chair to be quite valuable. And if you do buy from SVS, they have a money back satisfaction guarantee.

I have also heard great sound with the swarm described above, but you may need more than four for a room your size.

In full transparency, I am a dealer, but I just started out and do not carry any subs in my lineup, so what I say is purely from experience and in no way trying to sell my inventory.