What do I need if I want to measure this MQA stuff myself to verify that it works from ADC to DAC? So far I have not seen any proper technical description of MQA, just advertising babble. But I know at least one DAC that claims to support it and I know it uses stock ESS Sabre DAC so I know exactly what the DAC chip is going to do with the data.
What is input of the MQA codec? Modern ADCs are delta-sigma design, so the native output from the A/D stage is most typically 5.6 MHz bit-stream that is converted to PCM using a digital decimation filter. The apodizing filters in HQPlayer are designed to correct the problems from these filters.
What is output format from the MQA codec? If it is for example 96 kHz PCM, then certainly there will be further processing before the signal is converted to analog.
All modern DAC chips are delta-sigma designs, meaning that PCM input needs to be converted to high speed bit-stream. As said earlier, DAC chips typically convert PCM input first to 352.8 or 384 kHz PCM using a digital filter of varying quality and then use stupid sample-copying (sample-and-hold/zero-order-hold) to take the rate up to typical 5.6/6.1 MHz speed for the delta-sigma modulator to produce the bit-stream for the actual D/A conversion process.
What HQPlayer is doing is not adding any additional processing to the chain, but replacing the processing performed by DAC chip with better implementation done in software. So HQPlayer performs high quality digital filters taking the sampling frequency straight to 5.6/6.1 MHz or even higher 11.3/12.2 or 22.6/24.6 MHz, without any quality compromising sample-and-hold stages and then converts it to bit-stream for the actual D/A conversion using high quality dithered delta-sigma modulator.
So if any further processing would negate benefits of MQA Mastering, then certainly that is going to happen in all modern DACs. If you really want to get rid of the filtering effects and want straight digital path from the actual A/D process to the D/A process, you need to use native DSD recordings. Any PCM will have processing at both ADC and DAC side and that is unavoidable.