When you say .... "I am not happy with the stitch."
please tell us what is it that you are not happy with!
My first thoughts, without knowing the answer, would be ...
When quilting your sample sandwich was it the same
... batting?
...... fabrics (top/bottom)
......... thread,
as used in the quilt?
BTW ... can understand your 6600 love. A few years back, I traded my 6600 in an upgrade, but of all the machines I have had, the 6600 is still my all time favourite!!