I have a VTL ST-150, and my understanding, from reading some posts on the subject, is that the 6550 and KT88 are considered largely interchangeable, but that the KT88 was designed to be a bit more stable in amplifiers. For myself, I have always figured that VTL knows its business, and I have stuck with the recommended tubes. But many tube rollers swear by the various KT88s. I think that the KT88EH tubes you currently have installed are considered by many to be one of the best "alternative" tubes for the ST-150.
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