See the replies to Securespark's "Copying audio onto CD ROM". You have much the same problem. Also ---
Get the best tape deck you can lay your hands on. Most commercial tapes use Dolby B. Most cheap tape decks don't have Dolby at all. If you play back a Dolby tape on a non-Dolby deck the high frequencies will come out too high. This can make up for the shortcomings of a cheap tape head but listen more carefully and you'll notice a curious surging effect in low level, high frequency sounds.
You're only going to put your tapes onto CD once so do it properly.