I've heard pretty good things about the 6000, so I think you're on the right track with that.
Personally I'm in a similar situation. I pretty much hate everything about the stock HU and I do want an improvement in sound. My plan is to go one step at a time though, because I don't want to start running wires and custom fitting things if there's no need. I'm going to start with the head unit and see how much that improves the sound quality. If it's still pretty underwhelming, I'll find some appropriate speakers. If you find good speakers and they have enough power, you may find you don't even need a sub depending on your preferences.
If you want to really find the best solution, you would likely be best asking a larger audience on larger Subie forums or even on car audio forums.