Upon further Googling, it turns out that this headphone port, and the hardware expected to plugged into it, is some special 4-connector system that is used by the OS to detect ASUS hardware.
Windows has either built-in or custom drivers for the port that understand this hardware and treat it special, and it figures this out because the hardware is not just a standard headphone jack. While it should work as one, there are related reports of this jack just not working with regular earphones in Ubuntu here and on the greater internet.
Canonical would have to implement this driver behaviour in the OS if they decided it was a supported configuration, perhaps via a feature request.
My initial musings cribbed from the comments continues below.
I don't think there is anything Ubuntu specific here, but we really don't know because we don't know what Windows or this subwoofer hardware is doing special (and, therefore, what Ubuntu could do different).
However, if the assertion is that we are using the ordinary 2.5mm "headphone" jack as output, then this is software controlled to some degree on all platforms. We can at least change the volume, and some drivers/interfaces will allow pretty sophisticated tone control all the way to faking spatial audio. Also, the assumption here is that we are not talking about the S/PDIF output.
AFAIK, the typical Ubuntu controls for the headphone port will offer volume and muting. There may be something that offers equalization or output presets. If someone knows more they can provide that as a better answer!
But, if the assertion is that some hardware, once plugged into this port, tells that port to change the equalization to the point of a low-pass filter, then there is something else going on. These drivers can detect when something is plugged in, but there is nothing I know of that would be a generic way of saying "I am a subwoofer" vs. "I am a pair of headphones".
Maybe this subwoofer hardware is special; perhaps it shorts one of the unused wires on the cable (assuming this jack supports more than a single pair of signal wires) to signal "special ASUS hardware plugged in" to the driver. If so, then the Ubuntu port driver stuff would have to know about this, and then would have to know how to equalize the signal in a similar manner. The obvious conclusion based on this question would then have to be that it does not.
This sounds like a losing battle, however. I'd look for (ALSA?) audio equalizers, perhaps with some presets, or equalization that can be made into a preset so you could low-pass the audio appropriately when this hardware is plugged in.