Most vintage were in feedback loops around 12ax7A circa Ampex, Magnecord, Scully, Presto, Stancil Hoffman, RCA. You need at least a 400mh playback head for good output into a direct coupled tube stage, preferably 650 mh but not higher. This puts the head resonance within the audio pass band and results in lowest noise. The other way is the use an input transformer with a very low inductance head; 4.5mh. This also yields good SN ratio. With the advent of multi track heads the closeness of the head stack put a limitation on the amount of wire they could cram into a head coil. It's difficult to make a 650 mh multi track head. The Studer C37 used mostly passive EQ with a minor feedback correction. If you had an NAB or IEC EQ test tape then you could make several of the resistors variable and then set the proper EQ. You also need to check the time constants and maybe replace a few caps. Cheers! James