In windows 7, if one right clicks on the speaker icon in the taskbar, clicks playback devices, opens properties for one of the devices tab, then clicks the enhancements tab, one gets a list of filters ("Bass Boost" etc) that can be applied to that sound device. How can I register my own custom filter here?
Please note that I am not an IHV - horrible hacks involving unsigned drivers, etc are acceptable, as this will be primarily for my own use.
If you're an audio IHV you register them with your INF. The reference pages for them can be found here.
Please note that to be used with Windows, sAPOs must be signed, otherwise they won't load into the audio engine. To get your sAPO signed, you need to follow the WHQL signing requirements.
With USB headphones, how to you know the problem is on the windows side? Maybe the electronics in the headphones is bad? So I'll suggest soldering a resistor into the analog side of the headphone cable (i.e., hacking it old-school).
I notice that the device in question has an inline volume control. Can you just turn it down from there to get it to an acceptable volume?
I doubt, given that you see a similar problem under Linux, your proposed solution would give the desired result even if you could get it working.