I have tried lots of KT88's through the years, but do not know what STR is. My preferance was the MO valve co Gold Lion. Nowadays I am using 6550 of Sovtek for the same purpose, in a ultra linear design.
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That is an old term that came from MI (music industry...a guitar amp maker). It stands for Special Test Requirement. If you see it used today, it may mean the tube has been tested at some particular voltage, but in my opinion any current production tube should be run at real voltages before you plug it into your amp. So I might suggest a smidge of marketing. This tube you are speaking of is current production most likely.