I would venture a DHT SET, probably the simplest tube circuit, would qualify in most cases as a true tube amp using your criteria. The fact that SS devices may be used in a given circuit doesn't necessarily mean that that it is in the signal path. So to me a true tube amp is one that uses an output tube to amplify the signal to the speakers whether SET, OTL or PP. One could argue an OTL is even a more purist design in that there is no output transformer in the signal path. As to which approach is more purist does it matter but only the end result? Each approach has it own inherent limitations in one manner or another. The other factor is the output tube used as this too can determine what we will hear.
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