In terms of system cost, the proportions in my system break down this way:
31% speakers
21% preamp
27% amp
15% source
6% wires
In terms of affect on sound quality, I would rank them
1. speakers
2. source
3. preamp
4. amp
5. wires
The gap between 2 and 3 is closer than the gap between 1 and 2. And the gap between 4 and 5 is closer than the gap between 3 and 4. However, if you do have really good speakers (like Salks) then any of the other components could be, in your system, the weakest link (and have the biggest perceived affect on sound quality). For example, I changed sources after I had a solid amp and speakers, so it seemed to make a very noticeable difference. If I had my current source and speakers but a mediocre amp then changing that out would have made me perceive it to have a greater relative importance in my system.
I would guess that I spent about 1/3 of the system cost on the final 10-15% improvement in sound (to my ears). I would guess that tracks with other folks' experience--I could have gotten pretty close to the same quality for a fair amount less.
