One more thing to consider is the tube equipment you are potentially going to buy. tube manufactures will run the voltages at different levels depending on the use and power they are trying to get. lightly taxed tubes will last significantly longer then tubes run close to their max. Small signal tubes like preamp tubes i've seen last more then a decade. I've pulled tubes out of old military gear that have been used continuously for 50-60 years. its all on how hard they are run and the application. of course original build quality, and that area modern tubes are making great strides.Like others have mentioned.
Glen