I have a Marshall DSL40CR. I have checked the amps bias 5 days in a row and every day it's lower than what I have set it to. I was told bias in 40 watt mode should be around 35. When I set it to that on the first day and played...it sounded excellent. I put it in standby and powered off. The next day I turned it on with standby on and waited around 5-10 minutes. When I went to play it sounded really off. So I decided to check the bias and 1 tube was at 30 and the other 33. I reset both to 35. 

The next day after that I had to set it yet again because it was 30/31. The next day after that the same thing lol. 

Would this be an Amp issue or Tube issue? Once I set the tubes to 35...plays and sounds great the entire time I play. However, if I shut the amp off and come back later the amp bias is always lower.