If you are producing .wavs, it was a mistake to digitise to .mp3, as .wav files are only as good as the mp3 - the small quality loss going to mp3 won't be made better.
It does seem to me that Mickster is picking up on what I did? You've not really identified the problem, but are trying to cure it, kind of blind. With cassettes the real issue is hiss, and being ancient enough to have been doing cassette distribution, some problems were managed quite well in the mix. Cassettes hiss - low tape speed, narrow tracks, inconsistent bias requirements, often switchable replay EQ -and of course, Dolby. Back in the cassette days, the hi-fi enthusiasts often recorded with Dolby B on, which boosted HF, and then turned Dolby off for replay which made it very bright (and hissy) and then used their 32 band EQ devices to tame the hiss, but leave important content. So a song with lots of drum kit cymbal work and hi-hat stickwork, and 12 string guitars and high femalke vocals would have the EQ settings to reveal the HF content, then slice it at the lowest point they could get away with. On a more rocky track with drums, guitars and a make singer, the cutoff could be lower, making a less hissy recording. Mix these together on an album and you'd have more or less hiss, depending on the track content.
Distortion, you are probably going to have to live with. Clicks can be nibbled out, and on some software even redrawn with a mouse to soften nasty spikes - but only some DAWs have a hunt, seek and repair distortion preset, and while it might work on an acoustic guitar, piano and voice track - add deliberate guitar distortion and it doesn't know how to tell wanted distortion from unwanted distortion.
I found a cassette of my first band from 76, and it is dire. Thin recording, hiss, and distortion. Trouble is it is a very 'open' recording you can hear everything in a rather clinical mix. I have had no luck improving it at all. Generic reverb fails because it blurs things that should not be 'reverby', and the hiss removal - by manual and presets makes it really dull.
Back in the late 70s this is a local band recording, and in about 1998, I had a short lived digital recording system A Soundscape, with two Mixtreme cards. It tried to rival Cedar that the BBC were using, but it never caught on. The cassette was digitised on a Tascam 112 machine that I still have that actually works, and is virtually noise free - but if you have something that shows you frequency content - there is very little above 7K - because the hiss was very evident. I'd have liked to have tried my old band's cassette on it, but I lost it years back in a clear out. Motto - never throw any tape away! Despite having virtually no HF content above 10K at all, it's a pretty good recording. The band are still working, but I don't see this album anywhere - I wonder if they've actually lost the tracks, or they're too noisy? I might actually contact them and give the the recording - for a 70s recording it still holds up I think.
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