megaclaw, the cassette adapter needs to be inserted in the "right direction" and I hope you have gotten one that you could adjust the head to align it to the cassette deck head. I had the exact same problem as you when I first started, the SQ was horrible.
I suggest you take out the cassette adapter, take a look and see if there are any directional arrow to guide you on which side of the adapter should be inserted first and if it allows you to move the head, adjust it to see if it improves your SQ.
Good luck.