Rate these on order of importance:

Any of these, plus amplification/preamplification can screw up the sound. If the system sounds bad, the only thing that is important is how you can fix it. Whatever is making YOUR system sound like crap is number 1 in importance and in every system sounding like crap (or to be more P.C., unsatisfactory) that is something different. So, I am not a fan of ranking the importance of equipment in a sound system or cost allocation in a system. The right amount to spend on a piece of equipment is exactly enough to achieve your sonic goals for the system and not a cent more! There are many paths to a great-sounding system and many paths are not super-expensive. Equipment matching and speaker placement are everything. The toughest trick, I think, is going to a stereo store, listening to a particular component in the store's demo system, isolating and assessing that particular component's sonic quality and its sonic qualities from the rest of the demo system and then determining whether and how those qualities will complement your existing system at home while not making a mistake in judgement!