Dear Vuluongchitam: IMHO your Studio 40 are good speakers and IMHO too it is on the subwoofer side where you have a " trouble ".
First, for listen music I don't recommend one subwoofer ( for very good reasons that you can read in this thread. ) but two units. Paradigm has very good options that can blend with your speakers better than the JBL but the word " blend " is the main subject here.
I take almost a year to stay where I'm today on the subwoofers overall subject inside my system, not an easy task where we not only need patience but good know-how about music and how music sounds " out there " ( live ).
Your speakers are " ideal " to be mated with subs: great combination if you take your time.
Please read what ( and other posts related. ) I posted in this thread that can put some " light " to you on the subject:
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&27&4#27
http://forum.audiogon.com/cgi-bin/fr.pl?eanlg&1117893153&openflup&31&4#31
Regards and enjoy the music,
R.