I don't know if there is a break-in period for the D-350 but it seems to really opened up harmonically after having it these last few weeks. It sounded good when I first got it but the last few days I'm hearing a richness to it that although was there before when I first got it, was not there in spades as it is now. I think it just surpassed my beloved Aguilar TH-350 in that regard which is not an easy task. I also noticed my WD-800 always sounded better after its been left on a while, tube warm up I guess. By the way I returned my WD-800 not because I didn't like it, I loved it but the D-350 is doing the job for now. Maybe next year if I get into a group I'll get another WD or maybe a TT instead, that would be a difficult choice. Either way I'd be keeping the D-350, I think there really may be some fairy dust sprinkled on these things.