Good call on the 6S19P. But's not linear at all. In fact, it's virtually useless for audio. And you are right, a triode-strapped 6V6 is a much better alternative. Gain is why I love the EL84 even though it's not as linear as the 6V6 or 45.
My original point was that you cannot judge a tube only by the datasheet. You have to measure it. Look at the 4th, 5th, 6th, 7th... harmonic.
Ironically, its bigger sibling, the 6C41, is very linear.
