The problem with ADC is SD switching capacitor architecture. Data sheet for CS5510 has some pictures. Input impedance is affected by sampling rate, and may vary from 45k . What is more important, it's not linear, they put limits : 47-Ω and 0.1-μF to 470-Ω and 0.001-μF, and 470 OHm series resistor is the maximum. No way 5.6k, if respect THD level.