I'm buying a used Fender vibrolux tube amp , and I'm not familiar at all with tubes technically , so any help about tubes generally is appreciated . (use as much scientefic/technical terminology as you want , I'm an elec engineering student , but unfortunately they don't teach anything about tubes in my college !)

I've read or heard before that an amp can be working with a blown tube , so I want to know if that's true (if a tube is blown , will the amp be still working with less volume or something, or what) , and how to check if a tube is working nicely or not.

Also , I tried the amp yesterday , and I noticed that as you go higher with the volume (over 5) , the sound gradually gets a little bit distorted (very very little crunch or overdrive) , puting in mind that's it's a one channel amp (no overdrive channel) , and there is no gain button at all .... Is that pretty normal from an amp like that , or does it mean that something not good is happening inside ?
I also noticed that the tubes doesn't have EXACTLY the same color when they're working (considering that the three of them are 6L6) , is that ok ?