I'm not sure what basis and comparisons you've made to make this statement. Being fair, while these are better than cheap computer speakers, and may be better than some cheap speaker bars, the drivers are very small and extremely inexpensive and in a cheap shroud. They are extremely entry level for what they are. Which doesn't make them 'lousy', and if you've been happy with them, that's great, but it's more a factor of not knowing what good speakers sound like and what you're missing out on here.
If they work for you, and you don't desire better audio, then that's fine, but being honest, the vast majority of inexpensive speakers that you would buy separately, from companies like Klipsch and Polk would be better, and then there is a long list of brands which are a lot better. A big plus? You own these already.
Throw out the sub. If the speakers all have standard spring clip connections, pick up some speaker wire from
www.Monoprice.com and then an AVR from Accessories4Less that fits your budget. Maybe pick up a inexpensive 10" subwoofer while you're at Monoprice, and call it done.