Cassettes... heh... yes, I did copy a lot of my LPs and CDs to cassette, for listening in the car since I did not have a CD player in the car back in those days. I had a pretty high quality component deck to make the copies on though, and used high bias media. That went a long way towards mitigating the treble roll-off issue. On the recording side, a decent deck, properly used with quality media, could give you essentially flat response over the entire audio range. On the playback side, you had to make sure the heads were properly aligned (misaligned playback heads were also a common cause of poor treble response).
You also needed to pay attention to Dolby NR (or lack thereof). Playing back a Dolby coded tape on a non-Dolby player would result in overly bright treble, while playing back a non-Dolby tape on a player with Dolby enabled would result in a very muffled sound. Dolby NR also tended to exacerbate any treble rolloff caused by sub-standard media, incorrect (too "hot") recording levels, or misaligned heads.
So yeah... cassettes could sound really good, but you had to work at it.
Ahh, the "joys" of analog...
Nostalgia isn't what it used to be.