I believe the RGB sockets on the back of your amp is for component feed not red, green and blue video feed. To be able to make use your component socket on your amp, your freeview box would need to able to output component video. And component video is not available over scart.
There are Freeview boxes that do output component over scart - the Netgem iPlayer and a couple of PVR's. But most don't, they only output composite & RGBcvS (and a few also do S-Video which the OP STB obviously does as that is what he is currently using).
AFAIK the only AV receivers that can switch RGBcvS signals are the Arcam receivers. Therefore to improve the PQ via your receiver you are likely to need to convert the scart RGBcvS signal to YUV component with a converter such as This.