Just bought a nuvista preamp, but its going to be a while until I can get my hands on it. Does anyone know what make of valve is inside it? Also, I was wondering if anyone can tell me if there is any audible difference between the makes of 6CW4 valves that are still available? I've seen valves by RCA, G.E, Sylvania and even Phillips. I did read somewhere that there is a theory that all nuvistor valves were made by RCA but they had a trade agreement with other brands to sell them under their respective names. Has anyone had a chance to test different makes?