Power Amp

Air filter, one of the worst offenders.

In a diesel/gas engine it will ruin a turbo and cause it to go through oil like a crazy too. Air filter.. yup...Under 100.00 usd on a HD diesel, 20-45.00 usd for a gas rig..

Second bad fuel filters (low fuel pressure)... Can cause the newer injectors to take a dump... they will leak (drip) or over fuel, and cause cylinder wash, again hi oil consumption, and a lot of premature ware, no lubrication...

Regards

You should check your line voltage if its running high that could take out tubes sooner. If you change your lightbulbs frequently that's a good indicator.

Turn it off when you're not listening to it.

you can measure the voltage to the tube  sounds like a bias issue 

Heater voltage too high.