From quality of the cut surface depends two fundamental parameters. The No-Load current(Primary Winding Idle Current at No-Load condition) and magnetic permeability. In case of bad cut surfaces it can be observe decrease of the magnetic permeability and increase the idle current.
Very simple test of the quality of the transformer core is to check with ammeter connected in series of the idle current when no load is connected on the secondary winding.I’m of the opinion that high no-load idle current is not so good for the all core system.
Тhe main problem in the C-cores is the quality of the cut surfaces.
So, my desire always been to build criteria for simple quality test of standard cores. In order to find a core that could give excellent sound performance.
I want to note that is not talking about professional test and control of parameters.