Due to the reduced tape width and speed, frequency response of cassette decks was measured at -15 or -20 db level. At 0 dB level the response barely reached 10 -12 kHz.
It is not comparable with open reel responses, that are usually measured at 0 dB.
Optimistically, the best cassette deck is 20% of an open reel at 15"
If someone is interested, please send a private message with the email and I will send a scan of the Nakamichi 1000 ZXL review of HFNRR September 1981 - the issue that published the test of the Quad ESL63!