Cassettes are kind of a unique creature. Dolby invented a lot of technologies that greatly improved the sound quality of cassette tapes. However, my understanding is that none of these technologies are widely used anymore in the consumer market. So anything you buy made after like 2000, be it cassette or player, will have terrible sound quality unless it's designed for studio use.
Is it possible that you used an old, higher-end cassette player to record the blank tape?
Is it possible that you used an old, higher-end cassette player to record the blank tape?