Subwoofer meshes well with 2-channel?

I hadn't really given it much thought before, but will a sub work well with a 2-channel system (when reproducing music) or does it just muddy things up? I have full-range speakers (Tyler Linbrook Sig's) that go down to 30hz or so, and have 2 7" Seas drivers. I had considered getting bigger speakers that have more drivers, but this would be a more economical option, rather than getting some huge 300lb speaker that requires a crate to ship on.

If this the case, is there a particular reason that huge speakers are common, vs. a smaller speaker and subwoofer combo?
To clarify, the Velodyne SMS will work with any sub you choose: a low group delay, high distortion model like Stanwal's REL or a lower distortion, higher group delay model like the SVS that Bob has recommended from time to time. Further, you can choose to use the SMS' internal x-over to filter the mains or run them full range per Stanwal's philosophy. Or, you can choose to try both approaches and see which sounds (and measures) better. I did so and choose to actively filter the mains at a higher point. However, my subs were chosen primarily for their small cabinet size (rather than optimizing performance) so I don't want to suggest that active filtering of the mains is generally a better approach, just that it worked better in my system.

I use dual subs (Dynaudio 12 inch) to augment my Dynaudio Temptations. I don't high pass the signal going to the Temps. I just add some low end 'filler' with the subs. It provides a much more realistic sound to things like kick drums, for example. The best way I can describe it is that it adds a 'fullness' to the trailing part of the notes. Without the subs, although the bass is good, the notes die too quickly to be realistic.

It is sooo much better with the subs, even though they are only augmenters (I LPF the signal to the subs at 40Hz).
The key is integration. For 2 channel I do not like using a crossover for the mains, but rather rolling the sub (or subs--which is much better) in at a low frequency (35 to 30 hz). Phase must be right which is easily calculated by the distance of the subs to the listener and the speakers to the listener (thus the subs must be the same distance or closer for best results). Continually variable phase is a big help.

We developed the sub-PARC to give the ultimate sub integration experience. Obviously, I'm biased here, but it is what we designed this beast for--and it is a beast. The amplifier took forever to get right because most amps just can't handle the demands of the LFE channel and still have the finess for a good 2 channel system. It also has parametrics which makes integration much better and gets rid of the issue of muddying up the sound.

Rives Sub-PARC
Ok, I made a statement few years ago and they all thought I was crazy. Now that at least one person shares my experiece with sub-phase problem I will repeat what I said then:
My main speakers are full range, VA Mahlers which are porter, and my subs are two rels, a stentor III and a stadium III. For months i tried to figur it out but i dont think i have any doubts now. By setting the speakers as large and augument them with the subs at 22hz and below, When I did that with the subs' phase set to 0 cancelled the speakers below 25hz while increasethe output above that frequency. For istance, when the phase is set to 180, there is a normal boost below 25hz and a decrease of output above that. I alway thought it was a phase interaction between the mains and the subs. Note that if you ask Sumiko which sub to add to the VA they will strongly suggest the stentor. Go figure...
I must add that it Is a shame because I like them both very much,speakers and subs, but just cannot use hi-level connection in my setup. I am thinking to replace the subs for months now, but I can't get over that.