The most important thing with a cassette recorder is keeping the tape heads scrupulously clean - a cotton bud and isopropyl alcochol does the trick. The first thing you lose with dirty heads is the high frequencies.

People witter on about using a tape head demagnetiser, but I think that is a little overstated... I have one, but didn't really notice the difference. I believe there has to be a serious problem with the bias to cause the heads to maganetise so badly as to affect recording.

One with Dolby C, or better still, DBX noise reduction is the way to go.