Adding a sub. - can i start with one?

Change the E to an S.

That is, the RSL speedwoofer will be back in stock soon.  Compact yet potent bass for $449.  Considering your budget, imagine how a DBA including SIX(!) of these would sound.  May well be very very tough for one sub to keep up with the effortless sound these would produce.  

Also, they have a wireless option.  And, this company has refused to play the inflation card, keeping their costs unchanged  for years.  A huge value.

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.