I can't say it any better that Viridian does in his first post. Assuming no miss matched components, I feel the preamp is the most important component to get right for a great system. The only caution I would give is not to be focused on getting a "tube" preamp. There's all kinds of tube preamps. Some are great, others are not. Focus on getting a good preamp, not a tube preamp. If you happen to pick a preamp that you like the sound of and it uses tubes, great. Solid state has come a long way. No reason to rule a SS preamp out just because its SS. I've had great results with both tube and ss.
Here's a few preamps that I know well and you may find in your price range (used). My first choice would either be one of these 3: CJ Premier 18, VTL 2.5 or an Ayre K5. All 3 are great, but they sound very different from each other. Others I would consider are: Older Sonic Frontiers, BAT and Audible Illusions. I'm sure some of the other posters can give some excellent recommendations, as well. If I can think of any more, I'll post.
Here's a few preamps that I know well and you may find in your price range (used). My first choice would either be one of these 3: CJ Premier 18, VTL 2.5 or an Ayre K5. All 3 are great, but they sound very different from each other. Others I would consider are: Older Sonic Frontiers, BAT and Audible Illusions. I'm sure some of the other posters can give some excellent recommendations, as well. If I can think of any more, I'll post.