Subwoofers - crazy upgrade!

In the early 80's, I was driving a set of AR9lsi with a Perreaux PMF-2150B. I thought surely I had low end. Then I found the Velodine ULD-18. DAMN! Not only did I find extra power for the AR's but it truly filled in the actual lows I had never heard before. 

  Gentlemen, if I may ask a somewhat lame question please. Stacking subs Vs subs that have multiple front facing drivers ( 2 or 4 ) in the same enclosure with a single amp, are they comparable? I’d like to run multiple subs, but I can’t afford to pony up for 4. Also any comments on the Audiokinesis  Swarm is appreciated. I’m currently cash strapped and hoping to do something with a $3K budget. Thank you, Mike B. 

On a $3k budget I think you can find plenty of options.

My local dealer has JL Audio, Kef, B&W and SVS to name a few.  
They all have options to get you into a two sub configuration. 
I know there are other brands as well.  I did make sure

to keep the size in line with my room.  I seem to recall

reading that two smaller subs can out perform one big one?

@m-db thanks for the suggestion.  Why did I think of that?  One of my sets of speakers (Aerial 8b) are on casters for now as I play with placement.

My speakers are ≈16 Hz (–3 dB).  I dont have tolerance info on my subs.  I used them with the Aerials and for LFE on movies. 

@gdaddy1 

Could you elaborate why the 6dB boost?

I could understand 3dB "boost" just because you are doubling the cone surface. Acoustically, both cones will act as one device. But can't understand what would drive the additional 3dB boost.

I did add a REL sub years ago and enjoyed the improvement. Later I replaced that sub with two Rythmiks F12 and it was even better. Later added another two F12 and was better again. The last improvement was implementing Mukti Sub Optimizer (MSO) for the 4 subs (placed in the 4 corners) with  MiniDSP 2x4HD and it was an improvement again. It has been a journey for me.