Now looking further, the waves of left and right channel look a bit different even - the right channel has a wave going through the wave at 1kHz. Not sure if it's the primitive scope or an indication of something potentially wrong with the one of the channels. Sound like I would benefit from better measuring equipment to figure it out!
I have RME ADI-2 Pro, but don't know how to safely connect amplifier to it nor what software to use for measurement and graphs. I know I need a good dummy resistor and such, will look into all that - pointers would be welcome!
If I can make it work, I can also measure the whole chain - from ADI-2 Pro as source going through MiniDSP SHD and D90 DAC to amplifier output fed back into ADI-2 Pro. Wouldn't this be the holly grail of end-to-end SINAD measurement?
With a an extra twist that it would be a loop starting and ending at the same place