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.
BTW...how 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
Dave