I have a Janome 6600 new to me in December. I did have an eyelash problem yesterday (first time) but I had forgotten to change the tension. I use the walking foot and do free motion quilting and have been able to use many kinds of threads: Superior: King Tut, bottom line, metallic, so Fine, Mettlar both polyester and cotton, YSL, aurilfil, I even used some Dual Duty Coates and Clark and some kind of embroidery metallic. I think the SoFine 50 is the easiest one to use.
I do test all threads and adjust the upper tension. I did just buy the blue bobbin holder as I know you usually adjust the bottom bobbin for machine quilting and this keeps you from having to do it.
With some threads (metallics come to mind) you need to slow down. I never have the machine in the top speed mode. For finicky threads, right below the middle mode.
This machine has been a tension dream for me.