You must be doing something wrong, or perhaps you are suffering from some sort of hardware problem.
While I can't duplicate this on a 200D, using the 8000 here I setup my DAW to generate white noise and to analyze the audio output. I also ran Friture in parallel. Under no conditions is there a problem detected.
Thetis was set up with absolutely no processing on TX or RX. Leveler, phase rotator, expander, CFC, both EQ's and COMP all disabled. Passband was 50 to 5000Hz.
On receive, no EQ's, no NR or NB. Passband was 0 to 5500Hz. I probably should have set AGC to fixed, but I don't think it would have made any difference.
While I trust the DSP and displays on Thetis implicitly, I wanted to obtain true, full duplex audio output during transmit. Since this is impossible with RX1, I set RX2 to use ADC0 (same as RX1) and disabled RX2 mute during TX. RX2 audio was fed via VAC2 to both Friture and the DAW (Reaper).
The receive signal was obtained using an Xtronic coupler that is located on the output of my amplifier. The amplifier was in standby. The 8000 was set to 25W output, however a quick look at 100W output shows no difference at that power level. I did not want to keep the radio in transmit at 100W for the time it took to obtain the screenshots. I operated into a dummy load.
On both audio spectrum analyzers spectrum weighting factors were disabled. The specific settings for Friture are shown.
As you can see from the screenshots below, the channel response through both TX and RX is essentially ruler flat. I used peak hold in Thetis to more clearly show the response inside of Thetis. Note that the apparently slow roll-off at the low end of both audio spectrum analyzer displays is due to the log scaling of the frequency axis.
Note that an external source of white noise is not necessary. In Setup > Test one can select many different test signals for transmission. The results are the same with the built-in white noise source.
Using the built-in signal generator, one can obtain a much more precise look at the spectral response of the passband using the swept tone mode and peak hold. That is shown in the last screen shot. Unfortunately neither Friture nor the other audio spectrum analyzer I used have a peak hold mode, but the Thetis displays show how perfectly flat it is. If you zoom into single dBs in the Y axis, you can see a very small ripple, but that is inconsequential. Note all of the above was using the low latency filters.
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