[Five]

there's not a lot you can do about this, really. you can play around with eq or if you're able try a multiband compressor. If the elements of the mix that are too low have nothing else happening in their frequency ranges that helps. Like, say, if you had a recording of just a bass guitar and an electric guitar you could raise/lower the bass since guitar is more of a midrange instrument. For your tape you won't achieve any miraculous results but will be able to improve the balance with a bit of EQ.