So the RGB doesn't work anymore?

Very odd
Does the TV only have one SCART socket that supports RGB? Do you use a SCART switch box which can have RGB enabled/disabled (& if so, could the RGB button have been turned off).
What sort of options does the TV have?
Maybe one of the XBox settings has been changed, & you haven't noticed? Perhaps a setting that is only viewable when the XBox is in NTSC mode (e.g. when in NTSC, the XBox allows you to select HDTV I think, perhaps progressive scan, I'm not sure).
With the mod on, is everything definitely in PAL mode? If you change to NTSC, are all the settings (in EvoX & in the M$ dash) normal (except for it being in NTSC of course).
I can understand you wanting to go back to the advanced lead - it's much better than the normal one!
Can't think of anything else, eddie