upgrade my sound!!

Two or more subs, but NOT in the corners.

The logic is a little different than you think. The most important factor is not the sub or its power or even how deep it goes. So no they do not need to all be the same manufacturer. Mine aren't even all the same design. Two are sealed, two ported, one isobaric! They work great because the main thing is lots of subs in lots of different locations creates lots of different peaks and valleys that all together result in real nice smooth bass. 

The effect is so powerful we had a guy with two sets of ordinary 2-way speakers trying to get the most out of them. I said take the two that sound the worst, put them on the floor pointed at the wall, throw a blanket over the tweeter so you don't hear that. He was amazed to find the bass response far better than he ever imagined it could be.

Not saying use small subs but am saying you can forget all the usual BS about how important it is that they be big or powerful or EQ or any of that. With subs how many is far more important than which one. 

I see the logic.  4 subs, one for each corner of the room.  Right now, that might be a bridge to far for me.  I’ll start with 2 and see how it goes.  Do you recommend all subs be of the same manufacturer?

The most effective, easiest and actually cheapest solution is four subs. The number of subs is far more important than how big or powerful or any of that. So no it is far from everyone is recommending it. Most are still stuck in the old school EQ one sub approach. A few now are starting to realize more is better. But then they get stuck in the rut of thinking stereo. Low bass is not stereo. Timing, etc, none of the usual stuff applies. Just get four, you will not believe. The last guy to do this PM'd me with, and I quote, "F---n f--k this is awesome!" Just do it. 

Yeah I’ve been intrigued enough about the 2 sub discussion to try it.  When I first got into the hobby about 20 years ago the conventional wisdom was that was unnecessary but it seems like everyone is recommending it now.  I won’t do it in two channel as I wouldn’t have the room and I’m very happy with that system now

Room acoustics uber alles.

Add 2 subs. Three will give you incredible smooth, tight, articulate and deep bass like you won't believe. Do not sell one, add two. Put each one a different distance from a corner.

Right now with one sub there are areas where the bass waves cancel creating a dip or drop-off. So you turn it up and this is what creates the boom. Running more subs in different locations moves these areas around until the overall response winds up being very smooth and even. Smooth bass is fast accurate bass. It will also go a lot deeper. Incredible improvement.  

I recommend a Stereo Pair of Self-Powered Subs. Front Firing, No Ports, located near the Mains for excellent Imaging. 

Very low bass starts mono, then becomes directional by either the not too low fundamental and by the bass overtones.

I'd say both.... people say another sub won't mean louder booms....rather a more balanced sound..... 

I personally have never used more than 1 sub.  I like your speakers and have owned an earlier iteration of them.  What is the goal to be achieved with the second sub?  Better bass for music or louder booms for action movies?