I noticed the same thing on my Pass X250,5.After 3 days being on,it sounded too bright and underpowered.The regular1 hour warmup sounds a lot better.
Will leaving an amp powered-up all the time...
lead to "cap saturation" that causes the amp to sound bright? A friend sez yes. A listening buddy of his left his Spectral mono's on for three months and heard a difference when they had a normal warm-up.