"The XKing still hasn't sealed properly (I put it on in July I think) and still has stans solution seeping from the sidewalls"
Yes I have had trouble with some Cinti's tyres, that some of the riders I look after where using. (something to do with to much silcone to help release the tyre from the mould)
The stan sealant wasn't sticking to the inside, (leaving a white skin so to speak) so after a bit of testing I found it was best to wipe/rub the inside of the tyre with a rag and Dot 5 brake fluid, then clean the inside with brake cleaner and a new rag. (think I even washed it with soapy water as well)
Then put an extra scope of stans and inflate,hopefully you notice it is better but you still need to inflate higher than normal running pressure and go for a quick spin, it should settle down after 2/3 rides (so might be worth taking a pump with you just in case it goes a little soft)
Also be worth checking the sealant level after 5 or so rides just to make sure there is some in the tyre and also to check if the sealant (white skin) has stuck to the inside of the tyre and if that happens all should be OK!