power tube rating
I have a pair of VTL MB-450 amps, each with 8 6550 tubes. They were retubed in June of this year and now they need total replacing again. I was told that the tubes burned out this fast because they had a low IP value so I'm looking for someone to tell me what this value means, and what it actually measures. The tubes had a value of high 40's - low 50's and it's supposed to be high 60s. I need to know what this value is just so the next tubes I buy will actually last me longer.
Another question regarding tubes. The stock tubes are Svetlana, but does anyone recommend trying something else, maybe the Tung-Sol, SED or EH? Maybe KT-88s? I found the stock tubes to be "hard" sounding, so I'd like something warmer but with a full bottom-end.