Subwoofers - crazy upgrade!

@cdtd 

If you stack subs, doesn’t that introduce vibration

into each sub, causing distortions?

No. Both subs should be in phase with each other and working as one unit. The longer the throw, the further the voice coil moves off it's center point and more distortion occurs. The speaker cone shouldn't be jumping around. 

Also the sub amp gets to relax. Less work, less gain = less distortion and longer life span. All good.

Years ago I was using a Shahinian Acoustics Contrabombarde stereo subwoofer.  This monster was two folded 16 foot conical horns in a package a bit smaller than the size of two footlockers.  The horns were double slot loaded. 

When I cranked them up I could blow insulation out of the attic thru the vents!  Pipe organs on them was something to experience.

I sold it when I upgraded the speakers because I couldn't afford to keep it.  A sad day for sure.  The guy who bought it already had one. He wanted two per channel.  I hope he had a giant room in a giant brick home!! I can only imagine what that was like! 

That octave from 16 Hz to 32 Hz really makes a difference when the recording has material down there.  The other thing I noticed is plucked strings have a very, very low frequency content and the folded horn would reproduce it clearly. 

Move the subs out of the corner and try something like this..

Rather than stacking, you can also get a in-wall sub for either sub 3 or sub 4..and mount it in the ceiling at the same lengthwise/widthwise location in image above. Ideally, you would want sub 3 at 1/4 height and sub 4 at 3/4th height and so on.. The idea is to remove some heightwise nulls by mounting a sub at ceiling height...If you stack expensive subs all the way to the ceiling, your wallet might lose some of its rotundness.

@ozzy  wrote

I have 4 subs, placed in the corners of the room. But I wonder if stacking 2 in the front would be better?

Anyone have an opinion on this? They are heavy, so to stack might be difficult for me.

ozzy

@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.