I just remembered to update this thread.
Soon after getting the Quad back, I had EXACTLY the same issues. When I collected the amp, I had asked what they fixed as they didn't replace the LDRs (which I had supplied), Greg (the owner) was quite vague and said something like "changed a few components" but nothing specific.
Well since then I have had the amp open and cannot see ANY soldering or new components whatsoever. It is my opinion that they did absolutely nothing to the amp and because it wasn't playing up at that point, they thought they could get away with it.
I am now in the process of changing the LDRs myself, I have had bad experiences with techs before and this is another one to add to the list.
I'm still searching for a good tech in West London if anybody knows one.
Once again, if you want my advice, avoid Guitar Aid at all costs.