You have to be careful going down the AI path as it is hard on tubes that are getting more expensive and hard to find everyday plus with it's high gain can be an amp matching challenge. There are plenty of top notch tube preamps that don't present those challenges. Another option that I have found to work great with Class D amps is the Wyred 4 Sound STP-SE. They are currently on Black Friday sale and you can buy the Stage 2 upgrade model for well under the top of your budget.