yep went for cassette adapter and MP3 player... started off with the mp3 player on high volume as this normally seems to be best, but it just made for massive distortion especially on the bass.
So turned the volume and bass down on the mp3 player to get a nice clean signal through the tape deck heads, then cranked the volume and bass back up through the BOSE headunit if necessary.
I'm pretty happy with the setup and its way cheaper than replacing the entire head unit, only thing i might do is upgrade to a Sony cassette adaptor, instead of the absolute cheapest one i could find on ebay.
As an aside, most of the problems from playing tapes are a result of the sound being recorded onto the tape, and then the deck heads picking the sound up off the tape - whereas adapters cut out the tape middle man so to speak, so they're actually a lot better.