Subwoofers - crazy upgrade!

@ozzy 

You should be ok with 4 subs. Apply some DSP and add some acoustic panels if you want to maximize them. 
 

@gdaddy1 

Staking your subs doesn’t really improve bass quality. You are basically exciting the same mode. You might get more volume but you lose on quality. Only one listener will benefit from the additional volume. 
This video might help:

https://www.youtube.com/watch?v=McN2AygDMtQ

I added two Rhythmik F12se subs to my system and have had much the same experience.  Much better base qualitatively, but it also had an impact on the overall sound.

@spenav  It's NOT about more volume. You missed the point. It is about better quality, highed fidelity bass.

If you're setting up a home theater then spreading the subs around the space would help to get more even bass at different seat locations.

However for audiophile music listening stacked subs are far better. Try a visit to the REL show room and take a listen or go to PS Audio and look at their double stack or look at SVS stacked subs. (which is what Ozzy could have)

No where in that video does he say anything about stacking subs or losing quality of the sound. It's all home theater stuff and irrelevant to what I said.

I was also blown away when I added my 2 Rel subs. Just crazy. 

Yep, properly integrated subs make a huge difference. It can be quite frustrating getting them dialed in, but once they click it's magic. Take your time and really tinker with your setup to try every possibility of settings and placement to get it right. I use dual Rythmik F8 which reach down to 15hz in my small room but also cross with my mains up at 100hz cleanly and can play higher still. Since these can cover that entire range easily, I run a high pass filter on my main speakers to ease the load. This system now plays so fast and clean, it's shocking.

Glad to hear you're enjoying @cdtd.