Mickey2

You can get the equivalent Gütermann thread in larger cones as in the smaller spools. I can't remember now, but the sew-all and I think the top-stitch are under a different name when they come in cones. Gütermann tend to be smoother and less dusty than other threads I have used.

20S/2 is two core thread in a jeans weight thread? (I'm not good with the weight and thickness numbers). When I sew with top stitch thread I always max out on needle size, it helps a lot with thick and densely woven fabrics. Sometimes the needle tip makes a difference, sharp or rounded tip. A worn needle can have developed a burr and drags fibers and pieces of thread. It's very important to match needle and thread size, especially when the fabric is a bit more challenging.

It can be an issue to have different upper and lower thread, but as long as you are careful with tension it usually works. Have you done a drop test or tension test for the bobbin case? The basic test stitching on two layers of cotton turn out fine? The 301 bobbin doesn't take that much top stitch thread. If you notice bumbs or unevenness in the thread something has gone wrong in the production, but when I have had this, it mostly acts up in the needle hole where the problem was relatively easy to identify.