Did you measure the frequency response of these tapes?
There is no such thing as a frequency response of a tape, only a pair - tape + recorder/playback machine. Tapes differ by their saturation levels, distortion, noise, durability etc etc, however almost any tape can produce a flat response with a correctly aligned recorder. There are recorders which can get a flat response over 20Hz-20kHz on any half-decent tape with help of sophisticated tuning systems. Here is a couple of graphs (taken with AudioTester
) . One is a family of frequency response curves for a modern Sony HF cassette, another - for a 35-year old Sony cassette. Except for the difference in saturation at high levels and a bit of difference over 20kHz the responses at -20dB are flat +/-1dB from 20Hz to 20kHz and extend well over 20kHz. And that particular deck can do the same with 99% of cassettes.
Cheers
Alex
On the graphs: -10dBFS = 0dB DIN (250nWb/m @315Hz).
Sony 2012:
Sony 1978: