I have used bargain priced thread too, lately it has been a type of two pack polyester from a local store that sells just about any nicknack for house and hobbies. It usually behave very well, the only downside to the slighly more fuzzy polyester thread is more dust and lint in the bobbin-feed dog area. The worst of case is when it gathers behind the faceplate, I guess it enters with the take up lever? Gütermann is smoother and with out the minimal fuzz some threads have, hardly any dust at all, even after hours of sewing.
My point is, I don't think the stitch problem is caused by the thread, but when you test sew for adjusting tension I would go for the same thread in the bobbin as on top. The aim is for a good general bobbin adjustment and later on you can get by with simply adjusting the nut on the top tensioner. It usually works well with various threads in the medium weight range, but many keep an extra bobbin case adjusten for a particular top stitch weight thread if they use often switch between different threads. Generally you should be able to easily adjust for different thread in the bobbin than on top with out much problem.
I don't think the behive spring will behave as it should with out the smaller disk text to the tension disks. That's probably why you can't get it to stitch properly. That's what gives it the even tension. The take up spring helps too, but evens out the pulling on the tread more than any thing else. A 15 should behave very well in this regard.