What kind of studio do you have? Is it a project studio at home or a professional/commercial one? I have a home studio, but I take a lot of my gear to a colleague's commercial studio (Pro Tools) and get fantastic results.
I use a Presonus Blue Tube preamp, and I'm extremely happy with it. It's only about $150 for a two channel unit, so if you got two and the rackmount kit, you'd have 4 preamp channels, and still spend less than what you wanted.
I also have a couple of the Behringer tube units such as the EQ and the Ultrafex. They;re incredible! Behringer makes a tube ultragain preamp that I've heard good things about, and if it's as good as the others, I'll be picking up one soon.
Those suggestions may not be as good Avalon or any other high-end manufacturer, but they work incredibly well for the investment.