I agree with Mickey, you should be able to use just about any thread in your 15-91. The key is fine tuning the tension to the particular thread(after matching the thread weight to the needle). These older machines are generally far less thread picky than newer machines. I do almost all of my sewing with vintage 50s and 60s machines and have yet to find a thread that my machines don't like. Coats N Clark, Connecting Threads cotton and poly, Mettler, Gutterman, Madeira, everything from the cheapest crap thread to solid gold spiderweb lol. I use a thread stand for cones, the spool pin on the machine for straight or stack wound. I also adjust my bobbin tension first, then my upper tension. I have no tension fears.
Now, all that being said, I know that sometimes a particular machine just doesn't like a particular thread. Darker threads seem to be more of an issue.
Cari