6600 here too and I use Superior Threads for quilting- King Tut, Masterpiece, Bottom Line in the bobbin (sometimes for quilting too!) I use YLI 60 wt for piecing, sometimes Aurifil 50 wt. Except the BL, all 100% cotton extra long staple fibres - to my mind, it's not worth the hassle to use cheaper threads plus the lint can shorten the life of your machine and/or increase your servicing requirements so is a false economy.