Speaker choice and set-up

Now I mainly listen to music, but occasionally I'll use surround sound. Right now I have a Yamaha HTR-5490 receiver, Denon 280 CD player, JBL HLS 620 towers, and 4 JBL N24's. I am debating whether to stick with my JBL HLS 620 towers and add a Harman Kardon HK12SUB or purchase a pair of JBL 312's to replace my HLS towers and NOT add a sub. I can only afford to do one becasue I'm still a college student..please help.. My main concern is that my HLS 620's are only 2 ways and I'm not sure if they will blend well with the sub and the JBL N24's as compared to buying the 3-way JBL's with 12 woofers and letting them handle the subwoofer duties as well...I should mention that I just purchased the N24's and haven't hooked them up yet..Any help for a confused college student who enjoys speakers, but doesn't have much knowledge on the subject would be greatly appreciated.

What is your specific budget for a subwoofer?

The HLS line is regarded highly among consumers in the know, and I don't think you're going to improve your overall sound by replacing them unless you want to move up into something higher than the 312s. Assuming you add a sub to the HLS 620s, you'll get a better overall sound than the 312s alone.

As for the N24s, they're fine speakers, but not great as main speakers compared to what you've got. However, they'll work well as surrounds.

The biggest difference is that the horns on the HLS speakers will differentiate them from the drivers on the other speakers. To get good imaging and soundstage, set them up according to JBL's instructions.

I'll second that the HLS speakers are pretty nice. In general I would think you'd be better off keeping the HLS and adding a sub - a decent sub is going to go lower than most tower speakers will.

As an aside, the only complaint I recall hearing about the HLS series was that the horn on the center speaker was loaded such that the horizontal dispersion was not very good, creating a narrow sweet spot. Some people just rotated the horn by 90 degrees to solve this. I used to use a set of the HLS410s as surround speakers until I got a whole new 5.1 speaker package, now I am using them with an old sub instead of computer speakers by running my laptop out through an ancient JVC receiver.