Quote Originally Posted by fatmarley View Post
I get the Impression that there Is some kind of synergy with valves and polypropylene caps because the two of often seen together.
Valve amps often don't need very large values of capacitance but they do need quite high voltage ratings. Polypropylene match those criteria quite nicely and polypropylene is one of the best dielectrics, hence their popularity in valve amps. I don' think there's any particular sonic synergy between valves and polypropylene as such.