@leewrigley That’s weird. An S-Video source requiring a comb filter sounds really odd, as the role of a comb filter is to reduce artifacts (such as dot crawl) when a composite signal is separated into luma and chroma components. One might think of this as a process that is fed a composite signal and outputs an S-Video signal (Y and C separated). A proper S-Video signal simply should not require combing, at least in theory.
That said, it is of course possible that the Koryuu has an issue with the S-Video output of modded PAL N64s. The ADC in the Koryuu does support a variety of shaping and combing filter settings, and it should be possible to make at least some of them to not only apply for CVBS signals. If you’re willing to experiment a bit, I can provide you a version of the firmware with tweaks to the filtering of S-Video.
If it is not too much trouble, could you take a short(ish) video clip of the interference you are seeing, please? A smartphone video is fine, as long as it is in focus. Seeing the video might make it clearer to me what sort of interference you are experiencing.
By the way, some N64 games (such as some from the 007 series) apply their own filtering to simulate film grain, which I have sometimes seen to be mistaken for noise (UltraHDMI captures showed conclusively that the “noise” was there in the original digital pixel data). Could you state which game(s) you are experiencing the bad image quality with, please?