It never hurts to be proactive. If you have 3 to 4 years on your tubes , I would say replace them.
Tube recommendations for my Rogue Audio Cronus ...
First set of big tubes were the original stock Svetlana EL-34's. After three years, I replaced them with a beautiful sounding quad of Genelex Gold Lion KT-77's. It's now three and a half years with the GL's, and I need to start thinking of my next set. What are your ideas ?