Adding a sub. - can i start with one?

I still contend that any single subwoofer that is doing subwoofer duty will not "unbalance" the soundstage. Now if you choose to run the xover up to 150-200 hz, all bets are off. If the sub is crossed over low enough, even running the volume too high will not localize the sub, it will only negatively affect the tonal balance.

That said, I choose to use two. In my opinion it gives a more even bass response, twice the output, and two only have to work 1/2 as hard. But you can get good results with only one.

Hey @lalitk - curious, how high are your ceilings? Are they also high like the 18’ vaulted ones like the OP?

The REL carbon is a wonderful sub, and much improved over the 510 in terms of speed. It’s a nice move for your room and space. That said, it’s not only about output power, but also the ability to move sub-audible waves - sometimes room gain and reflections amplify this, and others do not. So seems the OP will have to test around.

Luckily the thread is not a “which sub should I get” or “Rel vs SVS”, and many of the sub companies offer a satisfaction guarantee.

If I had the funds, I’d get two Rel G1 Mk2 for my 18’x23’ room with 18’ high sloped ceiling (that is my listening space). With the 510, you also need more time as phase is managed by a pot in the back of the sub and not from the listening chair (the G1 has a remote for this). But unfortunately, time and money often factor into “the right decision”, despite it not resulting in the absolute best sound.

Started with one sub. A REL Carbon Special and yes, amazing first impressions. However, it only took a day to realize I could not fully integrate it for every type of music. I was constantly up from my seat to adjust gain. It was short lived experiment because I didn't listen to people here. I had to learn the hard way. I stepped up to the REL 212sx and it was a different story. I could get by with just one. I do not know if it is because it has four drivers, but I was able to quickly set it and never had to change it. I now have two because being type-a, it bothered me that I had bass information from one of two channels. It never "felt" complete or the "best" it could be. I added a second 212 and I will never go without. Magic, not just in tight, fast bass, but especially in midrange extension. The lesson here is that two are better than one, but the right one may be all you need. Happy hunting. 

@izjjzi I tried the 212SE in my room and it is very good. I had it set to about the lowest crossover frequency, and I could tell the soundstage was wider and cleaner on one side vs the other. I could have lived with it, but I would definitely benefit from two. I hear the 212SX is even better, and if I wanted to go through and spend the time integrating it, that would also be a contender.

@blisshifi The 212sx sounded amazing out of the box. The pair are actually difficult to adjust so not to be integrated. Took me a few tracks to integrate the first and a few tiny adjustments to get the pair in order. Placement was also far less a factor with only small changes needed to gain alone to satisfy. Highly recommended as for me, these are practically fire and forget subs.