The fact you have just had the wheels changed without much doubt it will be wheel balance. Take it back and complain and get them to do the job properly.
I've just had the same issues. Picked up a fresh car, the MAIN DEALER fitted 3 new tyres, crap ones
got wheel wobble, plus feel it through the floor, and strangely it was more noticeable when the engine was under a slightly increased load. So thought the worst like drive shafts etc....
Anyway the MAIN **** DEALER rebalanced the wheels, made a small improvement, but seem to get worse after a day or so. (think some weights fell off)
So went to a tyre centre I normally use and paid to have the job done properly (£16 in total) Each wheel was about 35grams out !!!!!
Now its near perfect, plus the car steers and handles better, well at least you can now feel what's happening on turn it etc, and it doesn't shake it's self into under steer when pushed !!!!
I will be contacting the ***MAIN **** DEALER very soon !!!!!!!! (SEAT by the way)