[font=trebuchet ms]Changing power tubes is like changing guitar strings. If you play a lot, and play hard, they'll wear out quicker, and you'll be able to hear the loss of tone.

Some amps are harder on tubes than others. My 1977 Ampeg VT-40 still has most of the original tubes in it, and it still sounds great. My 1976 Ampeg VT-22 had a dying set of Sovtek 5881 power tubes in it when I bought it 4 years ago, which I replaced with a new set of GT7027's that died violently within 6 months. (I think GT really stands for Garbage Tubes). It now has a set of JJ KT88's in it that seem destined to outlive me!

Marshalls seem to show the same kind of variability.

Note that the preamp tubes should last for many, many years. You're more likely to change them out as an experiment to see what different makes sound like, than because of tube failure. It's the power tubes that have to be watched carefully, and the amp should be set up (bias adjustment) whenever the power tubes are changed.[/font]