I have had my 15000 for 5 years and I love it. I have been using size L pre wound bobbins for embroidery and have been pretty lucky. Occasionally a bobbin acts up and that one goes in the garbage. I recently learned that I should have been using size 15 pre wound bobbins. Is that what y’all use? What is the difference between size 15 and size L?

I've been out of the business for 10 years now, but as I recall size L is shorter than Class 15. An equivalent would be size A.

Not all L bobbins and size 15 are the same.  I’ve always found it best to use only Janome bobbins, supplied in the Janome blister pack.  I did try size L prewound bobbins and the are slightly smaller than a Janome bobbin, they bounce and clatter in the machine.  I have used Janome prewound bobbins, they had a better quality thread and less prone to causing pick up issues when half full, the thread in the non Janome bobbins tended to pull back and curl because of the nature of the thread.  Janome thread doesn’t do that it’s good to the point at which the machine warns it’s getting low.

To enable the use of L prewounds, you can use a Bobbin Genie or silicone washer of the correct size under the bobbin, my dealer recommended this many years ago.  I purchased Coats prewounds in bulk in more than a dozen colours and never had any problems with thread quality, twisting or any other problems.  I have also used them (top and bottom) for sewing lingerie because the thread is so fine and strong.  Lastly, prewound thread makes beautifully fine FSL (lace) on your embroidery machine.  Hope this helps?