Way way back, when I used ZX Spectrums a lot, I used
to play dodgy tapes
through a guitar fuzzbox to bring up the level and square off the peaks
a bit. It worked, sometimes.
I seem to remember at least one circuit in the UK electronics magazines
that did something like this to improve the reliability of loading of
cassettes into home computers. IIRC it was a few op-amps and not a lot more.
-tony