It takes music to open up a tube preamp/amp

I'm wondering if anyone else is having the same experience with their tube gear... I have a CJ PV-14L and a MV60. A number of times now I have turned on the gear about 30 min. before I sit down to listen. As it states in the manual.. the sound will improve after 30 min. I initially thought it just need for the metal inside the tube to get warm to lower the resistance. I'm now thinking it takes 30 min of music... I've been digging up some old music I have not listened to in quite some time. For the first few tracks of the first disk I keep saying to myself "I guess this is not a very good recording after all". Then after about 15 min I'm saying hey this ain't so bad... after another 10-15 I'm saying dam this is cool... So tonight I went back and played the first track and sure enough it sounded way better than it did the first time. So are my tubes warming or is it just my brain ?