ASUS has a fairly well-known range of PC internal sound cards called Xonar Essence, but they have never sold all that much because add-in boards, in general, haven't had a strong market due to integrated audio processors in mainboards.
Still, the market for add-in or custom-made audio products will never actually disappear, just like the one for auxiliary audio devices will never go away.
In fact, it is precisely an auxiliary audio device that ASUS has just released, called Xonar Essence STU.
Technically, it is a USB external digital-to-analog converter (DAC) and headphone amplifier. In layman terms, it is an external sound card shaped like a small box.
"We received considerable positive feedback from customers regarding the excellent sound quality of Xonar Essence ST/STX sound cards, which led us to develop Xonar Essence STU as a USB device that offers even more premium audio quality plus easy controls and connectivity to notebooks," said Ives Chiu, audiovisual product manager at the ASUS Multimedia Business Unit.
"This is another example of ASUS listening to the community and acting on consumer demand."
The device provides asynchronous USB audio and swappable Op-Amps for DIY sound modification, and a 120dB signal to noise ratio (SNR). The headphone amplifier is very strong as well, of 600ohm.
Then, there is the precision clock tuning technology and asynchronous audio transfer, which reduce jitter and optimize sound fidelity.
Sound balance and richness is further strengthened by WIMA FKP2 and Nichicon Finegold capacitors.
Customers can even switch between low and high gain settings, "settings, accommodating better listening experiences across a wide range of headphones."
All of these things and more (dual volume controls, buttons that make software commands unnecessary, etc.) are available on the Xonar Essence STU. Check with online retailers or local brick and mortar stores to see if it is available for sale.