Sub. with full range speakers ?

I added a DD15 to my wilson WP7 speakers. Nice improvement. Then I added a second hooked up in stereo. Big difference! May work the same for your speakers. I know that some say that the rel is more musical. However, the computerized room correction and control of slope and point of crossover provided by Velodyne is quite important.
Otherwise room placement is even more critical and the need for a second sub required to cancel out conflicting waves.

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.