On Rel subs: Which are most musical?

It appears that rel makes some of the more musical audiophile quality subwoofers.
My question is which rel model do I go for? I am looking for a sub that isn't too big, is not sloppy but tight, blends in and fills the room with a natural sound that makes it sound as though it's live, in a very articulate realistic sounding system.
For what I am looking for do I look for a big size driver such as 15" or a smaller driver such as a 10" or 8" driver? Does a two driver sub do better than single?
What are the pros and cons between the two?
Does a smaller driver give me tighter bass and a bigger one give me lower and fuller bass?