Noise such a crackling/popping can indicate a tube going bad but it can also indicate the lack of proper pin contact in the socket and/or a build up of corrosion, dirt, etc. Some times just removing and replacing the tube will solve the problem as you will scrape off enuf of the crud. Other times you might need to clean the tube pins and sockets. Tube noise that sounds like a hiss is meaningless if you can stand the sound. They just are not candidates for positions where low noise is essential, as in phono amps, and some high gain line stages.
Apart from the obvious, the sound of an aging tube is usually dullness, and it can creep up on you. I would suggest that you keep a spare set of tubes around, not only for emergencies, but, depending on use I would periodically change them and se if there was a sonic improvement. If not take them out and try again next year (or whenever).
Oh, another thing, something I experience in one of my amps. There is a very lignt crackling/popping sound for the first 5 minutes and then dissapears 'til the next use. T think that this is due to the tube pin sockets are a bit slack and the heat up process expands the metal and makes a better contact.
Hope that helps a bit.
