is a subwoofer for music a good idea??



i have standmount speakers which go down to about 45-50hz.i feel my system lacks low end punch,specially at lower volumes. now i need to know if:
1. a subwoofer for music is a good idea in the first place
2. will buying a budget sub($400) really add sufficient low end
3.is there much info in music below 40 hz
4.will i lose imaging if i add a sub to my 2 channel set up

all you experts out there, plese help.
dazedandconfused
thanks for the great response ,folks!i'm new to this forum buisness,but of all the forums i've visited ,this is the most active.way to go!please keep the opinions coming to help me decide.for those who may be interested , my system is :
cadence arita bookshelves(i would be surprised if anyone in this forum has heard OF them,leave alone heard them! but they are really high quality stuff!)
rotel 971 mk2 integ.amp
rotel 951 cd player
kimber 4PR cables(the cheapest good quality i could find)

additionally,there is a offer on the REL quake,REL Q150E and DEF tech prosub 100TL.any of these any good for music?
all opinions highly appreciated,guys!
If you can get a good deal on the Q150E it might be worth going for. However I still insist on a home audition. Can you manage that even with the special deal?

On the other hand check out the reviews of the Q series subs by going to the REL website at www.rel.net and see if you can relate to it. Essentially the Q series are all same albeit power differences. Quake is not a good idea though. Too small and for computer application as its main forte.
Quadophile-

Have you hear the Quake sub? In a small room it is very effective. I don't know how many folks are buying $750 Rel's for computer systems, although, I am sure there ae a few. If you have a very small room the Quake is the best I have heard. This does make it somewhat limited in most applications.
Jla,

You admit about its limited application yourself, that is the only reason I was not in favour. If in future the system is shifted to a slightly bigger room or if the system is upgraded the Quake will be inadequate for music applications. Quake will depreciate in value much more compared to Q150E. There will be buyers for the bigger Q series but fewer for Quake. Is it not a consideration?

To answer your first question, no.
Being limited to cerrtain applications vs. being too small and for computer systems are entirely seperate issues. For small rooms I would go with the Quake. I listened to the Q150E and found that the Quake performed better in my room.