Being an ex-bassist{I focus on keys and vocals now, although I do casual sound tech work for some of the locals}, I have to agree that a lot of people try to play too many notes on a bass without stopping the previous one... Also many inexperienced bass players mistakenly assume that its the same as playing a guitar or a keyboard.... you see a gap and you fill it.
Unfortunatly this is not ideal with a bass, the bassist is the grounding member of the team, and the KISS method works best, KEEP IT SIMPLE STUPID. your bassist is either over playing and making things harder for the team and you as a whole or he doesnt really know how to tighten up his sound... if you can pull his hand back to the bridge then you'll improve it straight away {no more floppy sound from the bass} next worry about the amp sound, 10" speakers work wonders in tightening sound but if your stuck with a fifteen, check the EQ, and get rid of any smiley faces... having alot of bass is okay but you want a bit of low mid and tops to bring out the definition...
even just working on those three things will improve the overall sound no end!!