Thanks for the explanation.
The IC-7300 uses 16 bit high-speed direct sampling ADC that can capture the RF signal.
Correction: Ziegu G90 uses lower cost 24-bit mixer type ADC, it is not direct sampling.
The greater the bits, the more the dynamic range(the less quantization noise from the ADC);
16 bit ADC has 95db dynamic range
24 bit ADC has 144db dynamic range
Correction: The Marantz 20B and 10B use two-stage grounded grid RF amp with ECC88 and they are not in cascode.
I find this very intuitive, visual explanation of I and Q:
http://whiteboard.ping.se/SDR/IQ