I have a Foreplay II sitting here that I did some work on. It had a nasty hum that I couldn't work out. Finally the C4S boards came out, and it started working right. Not sure what the problem was, but it did not seem happy with those installed. Works fine with two resistors though. One thing about the way they did the heater to ground reference is sort of problematic though. Its run off one end of the heater string instead of being balanced. I changed that with a couple of 100 ohm resistors to make a virtual center tap. I did that at the same time as the C4S boards came out so I don't know if it would have hummed without it. It did hum with that setup and the C4S, but it was less than the stock setup. I also just have general concerns about the construction of those things. The whole wood box not being shielded thing is a big one, but the fact that I can see noise on the scope if I get near the tubes is another. They really should have used shields on the tubes. A cap from circuit negative to the grounded metal plate wouldn't have been a bad idea either. I may have to experiment with that before it goes back home.