I agree that King Tut thread is wonderful. I've never had problems with the YLI variegated threads. Sulky Blendables is the one that gives me fits when I'm doing FMQ. However its just fine for grid quilting. I finished a 44" square flannel baby quilt with Sulky last night, did a 3" diagonal grid. I quilt using various domestic machines.
I've been machine quilting for over 20 years. My quilting machines are: Berninas 930 & 1031, Husqvarna Viking Sapphire 875. The piecing machines are mostly treadles, I love Aurifil threads for piecing.
Sharon