Rennie -- my Janomes all usethe Class 15 or A bobbin. The L's are just a little shorter -- the 1600p is the only Janome that uses the L. The Innovas use the M.
I use prewound bobbins most of the time for piecing -- I just make sure to get the 15/A's. I have heard of some Janome owners using the L's with the magic genie washers, but I have not tried it. I usually wind my own bobbins when machine quilting since I like the thread to match if it pulls up.