Sub. with full range speakers ?

I use sub for my Vantages wich go down to 35hz. It only helps for really low bass and the lfe track in movies. I have been adjusting the volume down over time and currently have it set at 2.5, not very loud but enough to tell on very low notes and nothing else.

Summits do a great job down in to 24hz so there is really no need for 2 channel. You may get some benefit for movies.

if your spk specs are correct and placement is good i concur with mrmikesmail post. 24 hz is damn low, i assume that figure is the 3db down spec mentioned. a good sub will reinforce this dip which is rare in music, but, the secondary harmonics from deep bass will feel richer. if you have the $ to blow you will obviously get a low boost but if it is really worth it is up to your tastes. is your space that large?

It will ease the load on your main amp which should help with power hungry speakers.

I'd suggest only buying if you can get a full cash return after an in home trial.

In most cases it's a huge improvement. It just adds that bottom end--better foundation and the imaging in all cases I've worked with improved as well. The trick is getting the integration right. Rel in general is a good sub for doing that last bit--below 40 Hz.