Nice to hear you sorted it out. It's relatively common for a machine to have the bobbin tension out of place, even the need to have it appart, clean and get it back in correct order. I have had bobbin tension out of position because the tiny pin that turns the dial were out of place, on another machine the tiny release pin that is pushed in by the presser bar lever was glued up by sticky old oil and grime and it interfered with the release and firmness of the tension plates. It's one of those basic things we discover when we go over the machine from one end to another (both a second and third time in some cases).
Best of luck with your new 301 :- )