George is right, it really depends on the speaker. There are really very few truely full range speakers out there (i.e. usable bass output to 20hz or below) that are contained in one box per side. Some examples would be the big Montana, von Schweikert, and Wilson speakers. If you happen to own something like this then you won't need or want subs, and a two channel unit like the RCS 2.0S is fine.
The Salk HT3 seems to do amazingly well for its size and single 10" woofer, with usabe in-room bass response to the mid-upper 20hz range. My Alon main channels are designed with an acoustic roll off below 40hz or so. The VMPS RM/X cut off at about 36hz in my room. I never measured the RM-40 but they were probably about the same.
There are significant advantages to separating mains and subs, if you can implement a proper crossover between them (preferably at a very low frequency like 40-60hz) and get the time/phase alignment correct. Two pieces per side, whether coming that way from the factory or created by adding subs to your mains, can give you the best of both worlds with maximum bass quality/quantity and ideal soundstaging. The caveat here is that putting subs in the corners will definitely potentiate any room nodes. Without DSP or parEQ you will be struggling to find the best positions for four instead of two channels! But with DSP or parEQ the subs automatically can go in the corners and then you can play with the mains to find their perfect spot. Frankly I'm very surprised Genesis, Martin-Logan, Alon/Nola etc haven't started packaging their flagship two piece per side speakers with a DSP solution. The Alon crossover supplied with my speakers is sitting in a drawer, replaced by a TacT RCS 2.2X which offers vastly better integration in every single way!
Mirroring what George said, I found that the VMPS speakers needed as much wall reinforcement as possible in the bottom 2-3 octaves and and placing them in positions agreeable to the bass basically killed the imaging. Had I kept them good subs would have definitely been added which would have given me first octave bass and allowed me to pull the mains out into the room where they could image well.
If you are looking to buy TacT, if you can afford it I would definitely recommend the RCS 2.2X. Even if you only use two channels now it gives you the ability to seamlessly add subs in the future. But even if you buy an RCS 2.0S and outgrow it you could always resell and buy a 2.2X if and when you needs its capabilities. The TacT gear holds it value well and is always in high demand on Audiogon. Same with DEQX. Behringer is dirt cheap even new!
