Congratulations, it's a favorite model among free motion quilters. I'm having much the same issues with my machine right now. A couple of months a go it was all set up to free motion perfectly, but since I have sewn all kinds of projects, different thread weights and bobbin tension needs adjustment. You are correct, tenion has to be just right for FMQ. I see some recommend needle size 90 to 100 for FM. Some thing is up with my machine or thread, because I need to have tension way up, like 8, or I tend to get lots of loops under. I am also practising even movements of my hands as well as a steady speed.
For top tension I think the quick test is; turn the dial from about 0 to 9 (or even 0 to 0), it should turn fine with out any fuzz. Next step is to thread it up and check that tension gradually increase from 0 to 9 by pulling on the thread, obviously light on 0 and tight on 9. Thread should release when presser bar lever is up, and tighten when down (make sure needle bar is not in release position when testing).
Bobbin tension can be a bit of a fuzz but a general recommendation is to set the spring to hold 1-1/4 to 1-1/2 oz (
this is my referance page). I can't remember for sure but I think Tammi here on QB recommended 1 oz, either way it leaves room for fine tuning and it has to be reset for different thread weights. I guess someone sewing leather use a duty thread, while quilting is often a nice cotton. Some threads are better than others, but I have practised with a bargain priced mercerized cotton thread and the result is nice. I've had more problems with sew-it-all weight polyester thread, though it might just be about getting tension right.