My KGSS had always-on bias and I left the O2 plugged in all the time, so warm-up didn't apply, but when I had a T1S, the warming up of the tubes made a pretty big difference. Also, at a meet, I compared two O2, both plugged in a SRD-7 Pro and using a Quad II amp, one that was in there for quite a while and one that was just plugged in and I noticed a slight difference. Slight difference in meet conditions means probably no difference in real life.