There are many contributing factors to the overall sound quality of car door speakers, but in this case it's almost certainly the lack of clean power. That 25W rating on the HU is peak. Really you're looking at less than half that for RMS. It might sound decent at low levels, but as soon as you turn the volume up it's going to distort and sound terrible. There's a lot you can do to improve the sound quality, but for sure the first thing you're going to want to do is amp them. You'll want a 4 or 5 channel amp probably in the 50-125W RMS range per channel (the 5 channel with have a separate higher rating for the 5th channel). The better name brands (and there better lines) often put out more than the data sheet suggests (for example, I have an RF Punch that does 70+ when it's rated for 50). Conversely, if you get the junk-tier amps off amazon or from a store they'll probably do less. I can tell you from experience that you don't necessarily need 100W per channel to get loud and clean. That amp on four speakers in a sedan got louder than I could stand for any length of time and still sounded great.
