etl....your going to lose some volume, however you are going to get a deeper richer bass...
along with a wider frequency range in most cases....
one important thing to look for when buying subwoofers is to look into the frequency response...the wider freqency response, the more it is going to 'fill' sound....
the best thing for you to do, would be to find out what fq range your speakers have...
say your speakers only go as low as 180hz then you wanna look for a sub that goes from say 40hz to 140hz...
that way...your speakers cover 120khz-180hz and your sub covers 40hz to 140hz...
if you dont have full fulfillment of frequency then you are going to get an appearance of an 'unfull sound'....
thats the short and quick version...
i already know im going to get criticism on my example...and for other sound buffs....its been a while since i studied or tested for MECP so im just giving examples, so that he can understand the concept...
hope that helps...btw, my credentials...i have installed at circuit city for 2 years and now about 4 months at best buy....
"Do Not Seek Praise. Seek Criticism."
"If You Can't Solve A Problem, It's Because You're Playing By The Rules."
"The Perosn Who Doesn't Make Mistakes Is Unlikely To Make Anything."
-Paul Arden