I dunno about tweaking it, from what i have seen Freeview has a superior picture Quality to Sky as the picture is fine with movies and Freeview, but the thing is, stuff looks ok on my 19" widescreen monitor, sure its not as big, but i am guessing its got a superior picture quality to the 37" HDTV because the resolution is higher.
I think were going to be getting Sky HD soon, do those boxes support HDMI because at the moment were using scarts and they seem to suck preety bad anyway.