@stevehardy1 - yes, this is typical for the PRE and DSD. I would wait at least 200hrs before thinking of being critical. My PRE took 400hrs. Oddly enough, I just bought some new tubes (6922) with zero initial harsh, thin… lack of burn in. Tubes make a difference and sometimes they don’t seat for up to 400hrs. Also, they can go from amazing initial to degraded in 100+ hrs… The great thing about tubes is now you have a variable that can be changed to affect the character of the music and for that I am grateful.
Burn-in approach - to accelerate the burn in I have listened during the day and evening I have kept the music on and plugged in head phones, essentially running the system for 24X7 @ 168hrs a week. This approach should get you to better in a week or two and very stable in 3 weeks. The benefit to this approach is the BHK250 tubes get half the life pulled because you’re turning the BHK250 to standby in the evening.
Here is a pic of the BHK PRE if you get adventurous with tube rolling. The BHK PRE top tube vent comes off with (2) screws and the tubes are exposed and very easy to change.