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I don't think you can boost the output from the scart lead - i'd have a look at the quality of the cables you are using and make sure everything is pushed soundly home. A ticking or cracking sound suggests your wireless may be picking up interference. Can you set it to a different channel? IS the scart output compatible with the sender i.e. RGB, Composite? The re should be a menu in the STB's user setups.
I'm doing very similar to you with the co-axial fed from the STB to another bedroom. I use a digisender device to allow me to change the STB channels remotely. I don't see any problems with picture quality, RF is never as good as scart, but you wouldn't really see that on a smaller screen. But do try an amplifier, if the signal is OK via RF to the TV downstairs when you only have one TV connected. Make sure the Y splitter you are using is designed to split, not combine signals, oddly their resistance may work the wrong way round for you.