While I do not disagree with member Mike on 100hrs of burn-in to hear change, something that puzzled me was hearing my amps at 100hrs with KT150s, and then hearing my same model amps and tubes at my dealer with 300hrs on the same KT150 tubes. A puzzling difference, some added smoothing at 300hrs. I went back and forth to my local dealer hearing the difference and then I simply put more time on mine and eventually realized the same result at 300hrs. Note: my amps and caps were burned in prior with KT120 tubes, and trying again with new KT150s - a tube my mono amps were specifically designed for. I was not sure about KT150s in the first 50-100hrs, now with more time on them I like them quite a bit and keeping them for sure now.
Once my KT150s got to 300hrs I did notice some added midrange coming through maybe as a result of the most upper highs rolling off (just a tad more) after 200,300hrs. Particularly at 300hrs for whatever reason(s). After messing with various input/driver tubes too, and once i upgraded to really great coupling caps in my mono amps, the KT150s are super nice and I now can realize their full potential. I'd say at least 72hrs burn-in is a must with KT150s before you start listening to them closely.