I am a new 6600 owner too (got mine just a couple months ago) and so far my darling handles whatever thread and fabric I throw at it - mine is definitely no diva! I don't really know but it sounds logical to me that you might see little dots on both sides when sewing 2 thin layers together. There's no place for the stitches to hide where they meet if you don't have any bulk there. Maybe a more tightly woven fabric would help hide where the stitches meet? I'm at work so I can't test mine (I generally do tension tests on little mini quilt sandwiches so I haven't seen this) but maybe I'll give it a try tonight with contrasting thread on just a couple layers of cotton and see what I get on mine. I'm curious now!