Hey Guys, just wondering if someone can comment on the reliability of the various brands of power tubes. Specifically, 6550s as found in the Ampeg SVT 2-PRO. I've used several brands (EHX, Sovtek, Tung-Sol) and have had mixed results. The problem is usually bad tubes, or tubes seemingly going bad after a short period, which usually results in the amp shutting down on a gig. Thanks.