Adding a sub. - can i start with one?

It is a simple test.  Turn off your mains.  Turn on your sub and set the crossover at 50hz and then play a number of songs...not much tone.  Walk around the room...still not much tone.  I've done it with four different brands of subs and it always ends up the same.

But, when played with the mains, the additional grunt and the additional ambience provides the illusion of a change in tone...but when you listen to the sub alone, crossed low, not much tone there.

That has been my experience...no doubt, mileage may vary.

I am fortunate I don’t need subs in my room but if I did Rel would be the way I would go. Good luck ! 
 

@blisshifi 

Looking at your system it seems a few G1 Mk2 subs wouldn’t be too much of a stretch. Impressive indeed! I’ll bet it sounds amazing ! 
 

Happy listening !

+1 on the idea of a SWARM, especially in your space and for less $$ than two REL's. 

@ronboco Thanks for the compliments! I decided to start a hifi business on the side, which requires me to invest in gear for customers vs investing more into my own personal system. If the business takes off well, I'll reward myself with new subs. :)

It's obvious to me who here is speaking from subjective experience and who is enamored with an abstract idea