I've read through a bunch of thread were S-Type owners are having the same problem, but really havent seen a fix.

I have a 2003 S-Type Sport V8 with 52k miles, I just bought the car and put aftermarket wheels and tires. I have a vibration through the steering wheel it starts about 55 or 60 mphs. I don't think I had the car at highway speeds before i bought the new wheels and tires, so I don't know if the problem was there before. I had the tires rebalanced, still there, they next tried rotating the tires, still there. We next tried a softer compound tire and it rides better, but the vibration is still there. They next check out the run out of all the bearing, hubs and all the front end parts, nothing showed any wearing. Today I had all four tires road force balanced and still have the vibration. The vibration does not go away when lightly touching the brakes.

Anyone have any clues to what this might be?

Power steering rack maybe?