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.

You can unplug it, and plug into your PC right away for media management, all without the need to find a cable! This I see as a nice to have feature.

I had the chance to briefly listen to the DX9 with the HD800s, and they sounded clean yet holographic. The mid-lows felt slightly fuller compared to the K19, outlining darker voices distinctly. However, I wish the sub-bass on the DX9 could kick a bit deeper in the lower gain modes, which would help brings out even more depth in the perceived stage. But to really compare I need to listen with more different headphones.

In practice the K19 shines with its PMEQ features, giving it an edge when it comes to matching different music genres, voices, and gear. Somehow I feel like there’s room for improvement in the stock tuning of the K19 which sounds quite conservative in the lower end with some pairings, with a bit of equalization, it can sound a lot more euphonic.