I am powering the amp via USB and plugging it into a laptop. Using a Dreamcast and a DC2VGA adapter, I am able to get it working with RGBHV. So the passive sync combiner works. But switching over to RGBS craps out. The image isn't the kind that just flips vertically. It is completely distorted.
@invzim, do I need to terminate the Csync signal with a 470r resistor? The PCB looks like this right now: