Most class A or class A/B amps and preamps need time for their capacitors to charge up and stabilize. The same for most tube devices as well.
Listening after your entire system was powered off cold for a week+
Have you ever had your entire system completely unplugged-powered off for a week+, turned it all on, sat down for a brief listen, and immediately became disappointed with the sound after a cold startup?
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With temporary amnesia, rediscovered again today for the 100th time there were no problems with my dac or tubes or any component failures going on.
@4-5 hours warmup, a radical transformation with the sound. Transformers on the dac, preamp, amps, all warmed up - - woah, here comes the missing tone, texture, soundstage, opening up nicely, and musicality. "There it is!". Email this link to self, revisit periodically as prescribed. Maybe an isolated case :)