On your 15 you should be able to quilt with most type of thread, very little fuzz with basic top stitching (in other words not free motion). Good quality is important though. Very important too is matching needle size to thread weight, up the size when in doubt, it can solve a lot of problems when there's layers of fabric and batting. Going for large eye needles can help, but a larger size needle shaft in general will help with tension and thread passing through the fabric. Model 15 is one of the better models when it comes to this, so don't give up on fine tuning and adjusting. It should give perfect results. Do the drop test on the bobbin case, set the top tensioner between 4 and 5, the same thread for bobbin and upper thread; test sew and fine tune bobbin tension until it's right.
The old straight stitchers generally accepts various thread weights and adjust easily; Husqvarna CB, Singer 15, 66, 201, Pfaff 30. It's usually not about the model but getting things right. On model 15 is a favorite for FM work (not to exclude any other model). Another factor is cleaning and oiling the machine, general maintanance can solve a lot of diffuse problems.
Last edited by Mickey2; 02-02-2019 at 07:03 PM.