Your Insignia speakers are exceptionally good performers for their price (which is now up to $90/pr). Many people have modified them to improve their sound even more. Since you already like the way they sound, this might be an option for you. A complete guide to modifying them is here:
http://www.gr-research.com/insignia.shtm
The author of the guide says that a modified pair of the Insignias will embarrass much more expensive speakers.
If you would rather not deal with modifying your current speakers, then yes, upgrading will give you better sound, especially if you are willing to spend ~$400 a pair. What receiver and subwoofer do you have?