EQs
I'm glad you used the word improve rather than solve a problem. Eqs can be an invaluable aid to improving the apparent "quality" of recordings; older or poorer, live or poorly recorded.
You are dealing here with older recordings which may have lost their sheen or may not have had that presence in the first place. I am in the process of converting a load of older cassette and reel to reel performances of mine to cd. In doing so I am trying to see if EQing these tapes works or not.
Your ear is the best judge of whether it works on not. The ear is also the answer...there is no perfect parameter or formula to clean up these tapes. It takes time and expermentation to make that decision.
There are a few tips that I can give you from my own experience. Make very small changes at first and listen. EQs can effect dramatic changes for a minute but longer listening can make the changes sound poor.
Each band on most EQs controls an octave in the sound spectrum. Decide where the boost or cut needs to be made and then slowly make trims. Hum is usually in the mid low frequencies, hiss and "edgy" sounds tend to the upper octaves.
One last thing. You are working with an original tape and you will never "cure" the problem completely...all you can do is to tweak it some to make it a little more pleasant.
I hope this is of some help to you. Good luck!!
