It's either your speakers or your music file quality. Most likely your speakers, in detail, tweaters. Tweaters and all the speakers located on the top deck handle most of your highs and some of the mids. I personally haven't look but there may be a second set of speakers here that handle the mids. I'm not completely sure about the specific door speakers but they should be the ones handling the lows and some mids. Most people think that bass is good quality music. You've got some great ears there for realizing that it's not just bass. Some low quality speakers boost decibels or bass to mask their low quality to trick people that they're good.
I'm not so familiar with your HU but before you go spending massive money on new speakers, if your HU has an equalizer, kinda looks like this; You can fine tune your frequency ranges. At your low end is your bass frequencies and mid frequencies and usually on your right are the highs. Basically, pump up some of the mids and highs to force your speakers to play more in those ranges and maybe get a better sound out of them. If they're complete crap speakers it won't help but it's worth a try before spending massive dough on speakers. Unless you're rich, go for it. I'm a not a dedicated audio tech but I tried to help as much as possible so take my advice with a grain of salt.
