subwoofers or not for bookshelfs?

Are subwoofers helpful with bookshelf speakers, or do they mess up the character of the original speaker's sound in your experience?
Subwoofers work very well with bookshelf speakers that are missing the bottom two octaves, ie...20hz-80hz. Smaller bookshelf speakers (above 80hz range) require sub placement very near the speakers because some male vocals would be reproduced by the sub....not a good thing.

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but many people say rel subs are best for music, so would this mean it would sound better than oem subs acoustically speaking?
To high pass the main speakers, I need a home theater pre/processor? What is good for 2 channel music?
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REL subwoofers are good subwoofers...however, they are no better than another good subwoofer, except to their owners. Pretty much true with any component.

The best subwoofer (or subwoofers) for your system should be based on... your speakers, room, and intended use...each opening a small or large can of worms....depending.

All of the advise you get (or read) regarding subwoofer placement is right...and wrong...again, depending. Why?... because some of it may work well for you in your room, and some of it will not. All rooms are not created equal. large is your room and what bookshelf speakers do you own? Bob mentioned {corner placement} as being "usually" the best place for subs...I don't agree at all with the "usually" part. I do agree that corner placement can sometimes work well with "some" subs in "some" rooms.

One of the reasons many audiophiles have a love/hate relationship with subs is because they fail to get them to integrate, ie... (plenty of bass but no tone). This is what spawns the often over used term "slow bass"

Slow bass is only a factor of using large bass drivers to reproduce freq's (80hz and above)...something a 15" driver may not do as well as a 10"-12" driver...depending.

Problems at lower freq's are... placement, room problems, poor damping, poorly designed subwoofer...ect

I have successfully integrated an REL Q108E sub with my Von Schweikert VR-1s in a small library. Through trial and error I found: one corner was best of all the positions, the REL uses speaker level line in, the crossover is as low as possible (40 Hz) with the monitors full range, and the lowest discernible volume on the sub. In my system I don't want to actually "hear" the sub, but just notice it when it is switched off. The lowest frequencies are more felt than heard. Good Luck.