of course the subtracting wheeze assumes you have the exact frequency right, and the tape hasn't stretched in places and the reel-reel is precise, with a clean capstan. But done in sections, it is doable. Just time-consuming and ultimately the best you can do.

Notch filters remove all at those freqencies you choose to remove. Given that music energy is going to be present from 100Hz up it is always going to disadvantage the result.

For speech I cut it down to 400 - 4KHz if it is the best I can do. It sounds like a telephone recording. If the contents are important enough, the message gets through.

Some of my worst efforts can be heard at StroudVoices.co.uk - but for that website - content is king. Quality just has to be enough. I have captured the moment and I can't get it again, usually.