Speaker size and room interaction

I recently got a Rel T5i and it sounds fantastic.  I just plopped it down, hooked it up and bam, great integration with a more solid foundation for the music.  It sounds really great.  I bought it with the intention of getting 1-3 more.  
Anyhow, I have also had a Rel T9i and Rel Sho S3 in this same room with results that weren't as good.  The S3 was the worst actually.  Over powering, hard if not impossible to integrate, directional bass, bigger peaks and nulls, etc....

I’ve wondered for a long time now if we should be placing more emphasis sizing our speakers to our room appropriately.  I look at the speaker / room interaction in much the same way as driver / cabinet interaction in terms of pressurization.  
Anyways, I thought that I’d share my experience with the T5i actually sounding better than the T9i and Ss Sho in my particular room.  I can’t wait to get one more, then 2 more eventually for a swarm setup
The T5 hits amazingly hard. Excellent little sub for sure! 
I'll be curious how you deal with sub-to-sub interference, etc. I would like to get another sub, but I've heard (a) that it should be identical and (b) that one can create additional problems by having them interact.
If you’re using the bigger subs in the room with the Fritz speakers, I’m not surprised you experienced overwhelming bass. Somewhat cube shaped with sheetrock? Wood floors?
I use a S3 in a 40x24 basement space with no boom issues. Enjoy the T5(s).
I wasusing the S3 with Totem Arros.  Horrible choice.  With just one click on the volume, it would go from too soft to too loud.

Don’t get me wrong, the S3 is awesome and superior to the t5i but not in my room.  I bought the S3 new from magnolia for $999 btw!
Another really strange thing is how well the t5i integrates.  Like I said, I just plopped it down and that was it.  And something else somewhat strange is that it sounds good in phase and 180 out of phase, both.  
This is my first downward only firing sub and I’m wondering if I prefer them.  
Anyhow, the T5i has thoroughly impressed me.  For a little 8” sealed sub, it is just fantastic.  At the $275 I paid for it, its probably the best bargain in my system.

Hilde, I would think that using multiple of the same subs is best but the swarm guys say it doesn’t matter.  I think the main thing is having multiple sources of bass output in differing locations simply to create less dips but I still don’t see why using 4 of the same wouldn’t be best.  Which Rel do you have?  I’ll keep an eye out for another one...
Anyhow, I am super stoked with my set-up.  It’s reaching the point where it is in the top best sounding systems I’ve heard and its mine!

I have the REL 328. TMR and REL themselves advised I get another one of the same make *even over another REL.* Then, there are folks here. This is the hobby-of-directly-contrary-advice and the you-gotta-just-try-it. A minefield for expensive mistakes.