To tell the truth, I always spend all my critical listening time on the front end. When a design is first coming together, I tend to listen to the machine and not to the music. You can really get crazy, listening for every nuance, but be careful to listen to source material you know. Some material has flaws that can throw you off, making a bad speaker sound good or a good speaker sound bad. So I never evaluate using new material, and instead use music I am very familar with.My design efforts sometimes require months of modeling, prototyping and testing and that's when I get most critical. But after they're done, and they test well and sound good, I don't listen to them the same way again. After a design is finished, I begin to listen to the music and not to the machine.