About the amp warming up: That might be it, but I've noticed that it sounds nice and tight when I first play (about a minute of warming up), sometimes it doesn't, and the same after an hour of warming up. It could be changes in room temperature, but the changes in power sounds more possible. We have so much shite in my house going on and off. There is always power outages on my street too, and the power company keeps complaining that our street uses too much power.
As for tubes, its also possible. I got my amp in November 05 but it was a floor model. The newer models made even before the model I have comes with a 4 button foot switch, to change the gain/channel/reverb/and effects loop on/off. Mine came with only the 3 button foot switch. Which means it was the older model. It could be a year old but it looked very clean. I'm going to change the tubes, this is a good excuse to get some NOS tubes ;D I've talked to other people and they said they changed to the tubes out and pretty much any brand of NOS seemed to make it much better.