I have been sewing on my "All Machines Quilt" for a week or so now using different machines.
I'm using the C&C Dual Duty HD HEAVY thread.
The thing I've noticed is that every 15 clone no mater what make or shape as well as my 15-91 with the end mounted top tension has require me to dial in more top tension to sew properly balanced stitches. In the case of the 15-91, a lot more. The clones just took one number more, the Singer, several.
Those machines with the front mounted top tension have needed almost no tension adjustments when using that heavy thread. This seems to be true no matter what brand or design as well.
Kind of a funny little idiosyncrasy of the 15 type of machine, I guess.