US-1x2HR signal quality question

Hello dear community,
I have the above mentioned usb soundcard.
I'a stereo cassette deck enthusiast and I'm dealing with several servicing issues, adjustments and checks..
While everything seems to be working properly, there's an issue about Frequency response measurements, in regards to dolby C in -20dB.
It appears a massive drop down (about 10dB's), in the range between 100 - 2Khz..
Its very difficult to be a matter of adjustment or cassette deck malfunction.
Most likely it has to do with the quality of the signals that are getting inside the cassette deck.
Something is driving crazy the Dolby cirquits.
Any opinion about improving the signal quality that getting out of the soundcard??
Thanks in advance!
Regards,
Fotis