I'd say the 2400 should produce a decent enough 2 channel signal for music, so it's either the speakers (most likely culprit for the "thin" and lack of bass problem) or if you're not using a sub coupled with them, they likely cannot reproduce a strong, clean signal much below 60Hz, regardless of the specifications.
As mike c mentioned, if you're not using a subwoofer, I would strongly consider this whether or not you replace your main speakers as it will not only improve your music reproduction with these speakers, it will greatly improve movie soundtrack reproduction as there's tons of LFE (low frequency effects) in many movies. If you do have a subwoofer in your setup, it may either need to be configured and calibrated properly in your system or it may be insufficient for your needs. Assuming you have one, let us know as Nick mentioned what manufacturer and model number it is.
If you are weighing the speaker upgrade option, consider your left and right mains should timbre match the center for movie/multichannel music purposes. With this in mind, speaker preference is very subjective, meaning what speaker characteristics one enjoys another may not, so this makes it a bit difficult to blindly suggest speaker options.
If you are located in the US, a couple of online manufacturers offer some excellent options for the money. A couple of these you may want to consider would be M3s or M22s from Axiom Audio, CBM-170 SEs from Ascend Acoustics and AV123's x-ls X-series speakers. All three of these are some impressive bookshelf speakers that, when couple with a solid subwoofer, will give you great performance with music and movies without breaking the bank. If you're more inclined towards tower speakers, the Axiom M60s have had great feedback from several people. Ascend's 340 series, while technically not towers, are close to full range speakers and provide a similar sound stage and AV123 will be releasing within the near future some of the x-ls speakers in a tower form factor for a very impressive price as well.
I believe all three of these companies offer 30-45 day money back guarantees (less shipping) on their products, so you'd be able to demo these in your own listening environment giving you a good idea as to whether they're for you or not. If you're not in the states, let us know where and we might be able to suggest some simiilar offerings. If you know of some other speaker manufacturers that you are familiar with that you enjoy the sound of, we can possibly suggest some places to find good deals on them or similar sounding speakers. Some budget limitations and size of the listening area will also help us in suggestions if you are considering new mains... -TD
