Eh hem!...Subwoofers... What do ya know?

REL, REL , REL, oh did I mention REL, and the post that started this thread is correct, you must first understand what bass really is, how it is distributed, and how to properly integrate it i to (your system).
Every system dynamics are different due to room size etc, and as mentioned before if you know where the bass is coming from you have set it up incorrectly, not only will a properly set up sub allow your main speakers sound like they are bigger then life ( not only bass but in mid range also), also learn what High level connections can do for you when it comes to a sub. I recently upgraded to a line array system for my 2 channel system (if I could post pics here I would) , and was simply blown away , granted They consist of 28 6.5" woofers and 38 3" Motorola ceramic piezo tweeters in each column , using one REL Britannia B1 sub (yes its older but works as it should), the sound is phenomenal , and breathe taking , speakers and sub are controlled through a Samson S3 crossover network, and of course bi amped . 
Subs rule , but IMHO buy from a manufacturer who only builds subs for the best quality and technological attributes towards your systems dynamics.
  

Correction 28 tweeters , mis type

Miller.
I have just ordered another ML Dynamo to match existing one.
If it does not play nice by wireless then I can wire it as the wireless transmitter also has RCA out.

By my room I really have to have them diagonally opposite as I said especially to keep well away from my tables.
I could possibly get to three subs but due to layout that would be it and third one would go left rear.

I will try the new one to the rear right but not exactly in the corner and play around a bit and see what improvement I can realize.

Not that I am unhappy right now but you usually do not know what you were missing until you find it!

It really is easy, go big or go home! Just get a Wilson Audio Thor's Hammer and call it good. Kidding. Lot's and lot's of good info here. In the end though, one way or another, a compromise must be met. Budget constraints, testing capability, room realities etc. Not all can have the perfect acoustic room. Like Eric mentioned, get what you can, dial it in to what sounds good to your ears and call it good. OR, go all in, have the room tested, do all the treatment, demo locations/subs and take that route. I recently eliminated my sub for my two channel setup. My room constraints wound up creating more problems than I could/want to try to "fix" so I binned it. Being a drummer I like the low end, but at the end of the day, once my ears changed a bit and the low boomy bass I "thought" I liked, was actually a very real negative. Anyway... I compromised and now I'm happy. (for now) haha!

I think everyone is on the right track relating to their own likes . I agree having lots of power to mains is important. My mains are 3 way bi amped . 150w solid state to the 15” woofer and 100wch tube to the horns, tweets. Asking the woofers to reach down to the basement takes away from their clarity . Subs reinforce those very low notes and in this way gives an omnipresent feel to the sound . With my remote on my c46 preamp i can cut the outputs i used to engage the subs . Sound is still beautiful but having that ability from your chair to turn them on and off sure trains you to set them modestly as to only give the full picture nothing more.  Think of the 20th century fox orchestra intro where the last sound is made by a very large drum kind of a whomp powerful yet subtle . Can you hear a sub? Or the room that drum was recorded. The silence between notes is the most beautiful sound.