Changing the amp isn't going to change much. It's fairly easy for any solid state, direct coupled amplifier designer to achieve flat frequency response from 20-20kHz. The speakers are not a demanding load so the Pioneer isn't being stressed unless you are expecting more volume level than the speakers and amp can reasonably produce. The speakers are though under $200 and very small enclosure types. They won't tolerate being overdriven as they will show signs of distress rapidly.
Other than that, "the sound is not very good" doesn't even begin to suggest what you are expecting or what you have.
It wouldn't hurt to do some experimentation with placement first. What source player are you using? If you are playing MP3 files, they suck when you put them through a more sophisticated system than $5 headphones.