actually that's a good idea. I will try with the headphones to see if I still have the issue. This will allow me to isolate if it comes from the DAC. I mean it comes either from the DAC or from the DAC - speakers combination because before with the asus sound card I did not have that issue.if I read correctly, no subwoofer and main cut at 80 Hz.
as long as you keep this cutoff at 80 Hz, you will have ear fatigue.
Use a headphone