I feel like it is the norm for consumer audio manufacturers to put the starting point at 20 Hz for the response specification, even though the device may still produce frequencies below that point to a certain extent.

It could be more ideal to either listen to the unit to determine the actual perceived intensity of the bass beyond that limit, or to measure the output from the combo directly!

They are quite different in tuning, with the Hifiman being smoother and more rounded in the higher registers, while the xDuoo is not as dynamic, offering a clean bass attack but placing more emphasis on vocal harmonics and enriching the mid-bass.

Ultimately, it depends on what you plan to pair them with and your music preferences to determine which one suits you better!

The 20 Hz limit, which most manufacturers offer, is a reason for me not to buy it.

My headphones go down to 6 Hz, and I can feel the bass at 5 Hz or 10 Hz, but not at 20 Hz.