so, i recorded a type 1, normal bias, on a 70s cassette deck, and it sounds FANTASTIC

Mix tapes on 8-tracks were The bomb.They mostly came apart when the splicing tape let go. Easily fixed back in the day.

My homemade 'mix tapes' from the 80s-90s still sound great on my early 90's Denon deck. 

I am surprised those old tapes still even work without jamming.  I am also surprised you aren’t getting “print through” on the tapes being that old.  

Cassette tapes were never designed for music.  Tape speed and size were for voice recordings.  Like lps.  Reel to reel tapes were designed Ed for music with limitations.   Dolby was a bandaid for the tape hiss.  Transistor radio to Walkman to portable cd players , mp3.  All compromises for audio on the go.