Adding a Sub do i go with 1 or 2 ?

astewart8944 is right about more taking care of modes, but while the research shows greater benefit from more, it also shows greatly decreasing incremental benefit from more than four. So what you want is four. Also they need to be placed asymmetrically, and somewhat out of phase. Do this and you'll get amazing bass. Read the Swarm threads.

I use a single REL S/5 SHO and sounds great...a friend has an almost identical system, but very different room, and he uses a pair of REL S/3 SHO and sounds great...so depends a lot on system and room...

Are the new S/5 SHO and S/3 SHO as good as the REL Stentors ?

Hard to believe how far off you guys are. Its not a question of which sub. Only when you get to four subs, properly located, only then does it matter which drivers you are using. As long as you stick with the old paradigm of one or two, you are so far off base it really does not matter what you use.

Insert gif of head banging into keyboard until turned to mush. That is what you’re doing.

Frankly I was more than a little shocked to learn the answer has been there now for well over a decade. Seriously. Go do a search.

Guess its like autophagy, induction cooking, or any of a number of paradigm-changers. Hardly anyone adventurous enough to adopt the new. Almost everyone exposed to newer and better lamely sticks their head right back in the same old feed bag.

Seriously. Go read the threads.

I use 2 RELs on the same (mono) power amp derived signal as that seems to allow for the standing wave/null issues to be addressed to some degree, although one sub can work fine when properly set up in spite of claims otherwise. A stereo sub arrangement won't put equal bass levels in each sub so they then can't help each other as much, and my subs are adjusted carefully and the stereo image from the mains is excellent. Much has been made of the 4 sub solution ideal and I like the "swarm" system's design goals, but if you don't have room for 4 subs you can still enjoy music although I recommend you not invite Millercarbon over anytime soon.