fastpedaller wrote:..... Certainly a Quality control problem somewhere, but I suspect very difficult for Spa to have any control over.
Why? Spa can do the same as anybody else and return faulty goods. It is Spa's quality control at fault and they should simply replace the frame. Packing it with coke cans etc is fine for a cheapo 2nd hand frame but not for something brand new from a shop with any reputation to maintain.
in which case by that definition any bike-selling emporium that sells any frame/bike that is made in Taiwan (to the usual standards) has 'no reputation to maintain' and should immediately return most of the bikes on their shop floor; similar problems commonly exist in most such frames, alu or steel. Accurately reamed seat tubes are more the province of handbuilt frames these days and unsurprisingly these do cost more.
Note that the OP initially had a problem with a seat pin that was definitely undersized and (IIRC about typical 'pro' branded seat pins) might have been relatively short so would (for any given diameter tolerance) wobble more for that reason as well. If a longer seat post of the correct diameter had been used from the start it would probably have stayed put and not been noticed either. It could easily have had about 1/4 as much wobble to it, or (if the bottom is very slightly squished out of round), effectively none.
If you demand perfection then you can either ream the seat tube and/or get a seat pin machined to fit. Several times I have done the latter because that was definitely easier than trying to persuade someone profoundly ignorant/obstinate that their frame wasn't going to break if you removed 50um of metal from the inside of the seat tube.