Do not make the mistake I made
Pre-wound bobbins from Superior Threads on metal are for appliquers; they do not fit Janomes
SuperBobs on cardboard are for machines - they last til the thread is used up, no problems.
I buy Bottom Line SuperBobs for my Jem Platinum and Jem Gold. BL is a 60 wt poly thread that lasts forever. I pieced a twin top with one bobbin. I am DEVOTED to them.
Be mindful of what others have already told you - L size is what you want for sewing machines; Ms are for Longarms.
I use Aurifil 50 wt cotton on top with the 60 wt bobbin; have for years, never a problem.
Bob has a page on his site that tells you which machines will take his pre-wounds.
http://www.superiorthreads.com