I think you made some good choices. I had the 2nd generation Silver S8's myself and really enjoyed them. I'm sure the newer version that you have will sound even better. Also, I think it was a very smart move to go with the better centre channel. I'm not too concerned with HT, but the one thing I see quite often is that the centre channel sometimes has a hard time keeping up with the main L&R channels.
As far as a sub goes, I would just wait a while until your speakers break in and you have a chance to get to know them. Doing it that way will make your sub selection easier.