I recall testing the Core bookshelf speakers against others, probably up to $500/pr or so and was very impressed, for the price, especially when Core's are on sale.
That said, they are an excellent value vs. entry to even some mid level speakers. But they are designed to be an easy load for any AVR to power, so an external amp is not going to be your solution.
Better sound, in this instance, will be provided by better speakers. My advice to you is go out and listen to as much as you can before spending a dime!