I bought a Xbox HD AV Pack

Which has 6 output jacks:
Component Video Y output
Component Video Pr output
Component Video Pb output
Audio-L
Audio-R
Digital Audio
and also bought a RGB Component Video OFC cable

I have enabled HD and digital audio settings in MS Dash and box is set to NTSC, so I know all that is fine.
The problem I am having is when I turn the XBOX on the flubber screen flickers and then when it boots to my Dash XBMC-SVN_2007-01-01_rev7469-T3CH the picture is very dark screen output is more than 480 or higher.
The obvious answer is to adjust the TV settings but I dont think the problem is that, as when im running as normal NTSC 16:9 on the which is viewed on YCbCr channel it looks fine, altough the Yellow in the RSS feed is abit duller XBMC, Games, Movies all look the right sort of colour...
When ever I output higher resolution I have to switch to YPbPr channel as YCbCr goes out of range... I hope im making sense and not confusing you. Do you think the problem is with the TV, Cables or the HD pack itself?
Also having probelm with digital audio, when I put a game on I just get a load of noise through my amp tried everything to fix this but dont know whats causing it... MS Dash settings are set to Stereo, Dolby enabled, DTS enabled.
I hope someone can help, as I would like to run XBMC at 720p... Oh TV is a Firstline 42" Plasma, it also has a VGA connector on the back would I get a better picture using that instead of Component cables?