I too have had problems with my Floriani Thread shredding while using my 15000.  I have lowered the embroidery foot and slowed down the machine, but still experience shredding.  It seems to be worse when the machine is sewing toward the left side and slightly upward.  I have also had this problem when I am doing free motion quilting and moving the fabric toward the left and slightly upwards.  It is like it starts to shred and then the thread gets caught in the bobbin area and it makes a big mess with several threads appearing on top of the fabrics.  I can turn the fabric while doing free motion quilting and I do not have the problem.  It seems the machine just does not handle sewing toward the left very well.