I love my SVT-2 Pro. However, it is sick. Its power tubes are finally wearing-out to the point where the amp is getting a bit too quiet and distorted. They are your standard issue EH 6550's. I've gotten about three years off hard use out of them--no complaints here. Now, I've replaced tubes tons of times, but I have an opinion-based question for all of you in the know about 6550's. In your experiences (primarily with all-tube SVT-type amps), what 6550's offer the most durability and longevity? Though tone is a wonderful thing, I'm more concerned with the long haul at the moment. Ideas?