Hi Exipnos,
The method I described is not necessarily the "best method". It happens to be a setup that I have personally experienced.
Of course, the purpose of using a sub in a home theater is generally to help out the other speakers that are incapable of powerful bass. Even most mid level tower speakers can't reproduce the powerful bass that a decent 10" and larger separate sub can create.
The multiple sub theory is based on, the more subs you have, the less work that each sub has to do. One dedicated sub has to "help" 5 or more speakers with the potential of 5 or more different "effects" coming from each. Adding a sub just for the rear effects channels gives the mains sub a lot less work to do making it more efficient. When we tried out using 2 subs in this way, we could not only feel extra bass coming from the rear, but could also feel more "pressure" as there is now more speaker cone surface area to push the air in the room. (Read Bob Carvers white paper regarding the design of the Sunfire True subwoofers). My friend uses one for the mains sub.
You could use large speakers for every channel. The only problem with it is that the surround speakers should be at ear level or slightly above. Other than mounting towers to the wall (bass would not be as good), the treble and midrange drivers possibly may not be aligned with your ear level.
If the definitive techs do offer heavy bass and can be placed on the floor in a way that they can be used for surrounds, They would work great.
Look at it this way. You can get speakers with the sub built in (Def Tech) or use a preamp and have separate speakers and subs. I prefer having more flexibility. That's how I prefer it.
Having multiple subs would be difficult to setup.
Since the Dolby Digital stream utilizes the full frequency range of 20-20k hz, every channel has the potential to produce deep base.
Since low frequencies are non directional. It only matters that you place the subs where room modes would not have be a problem. It really doesn't matter where you put the subs for the satellites.
Darren
BTW, I would like to here those Def Tech speakers. I am going to try to find them at a local dealer.