Subwoofers - crazy upgrade!

This is a little excessive but it gives you something to dream about when you’re talking about stacking subs. https://share.google/PGq3mBI8pqe3sAbXq. 
 Now that you have discovered bass. Don’t overlook the need for room treatments, it too can be just as transformative. Good luck 🍀 in your stacking journey!

@ervikingo  Not friendly to my back.  My speakers go really low (REW shows them starting to drop at 15Hz) but I do have 2 subs on standby!

If your using -6dB rated sub-bass woofers read no further.

If your sub/s are rated at -3dB@xxHz, when placed on a small harbor frieght dolly using an inexpensively long interconnect from Blue Jeans or Monoprice Cable one can easily be rolled over to your listening position.

Repeatedly playing a downloaded low frequency cadence (not music) while slowly strolling around your rooms perimeter you should be able to easily locate and map your rooms standing waves locations where each sub can be positioned for their optimum presentation using the least amount of gain and EQ. 

When -3dB subs or and full range speakers are located out in most rooms null areas no amount of amplifier power or rated ability can overcome the physicality drain on the low frequencies by the null position.

This out of positioning has been the ongoing issue nurtured by either the manufactures lack of knowledge or a marketing stratagy towards the unique home audio market. 

PS: To reduce enoying low frequency born throughout your dwelling leave them on dence foam topped dollies. Take care of that back. Enjoy.

I use a mixed setup: 3 Klipsch 14SW connected to a Denon 4800X (which has 4 separate subwoofer outputs). The AVR isn’t connected to the main speakers and uses Dirac bass control to correct the subwoofers only. They are positioned on a triangle (1 center front and 2 rear at the corners) and the bass has no dips or reverberations.

@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.