The only software way to do this would be to either buy a soundcard which has an EQ with it OR then you have to have the following setup:
- Virtual Audio Cable (costs a little)
- A VST host software, I've tested VSTHost and recommend using that
- A VST Equalizer, the best free EQ which I concider very good for that is Electri-Q Posihfopit edition
Virtual Audio Cable works like an output device which seems to be a lossless "stream", I can confirm it sounding very high quality, I was very doubtful that a 3rd party software like this would be able to sound great like that but yea I don't hear this sounding any different than using WASAPI in foobar2000 really.
To get it working do following:
1. Put Virtual Audio Cable as standard sound output device in windows control panel
2. In the VSThost program you can add the VST EQ between the "Engine Input" and "Engine Output" and whatever other VST plugin you may fancy
3. Go to Devices -> Wave and select MME: Line 1 (Virtual Audio Cable) as input and your normally used output device as the output port.
4. Tweak the buffer setting on that same config window, the lower you can use the lower latency, I'd try there around 630 - 525 if possible depending on your computer, if it's set too low the playback will sound distorted (like clipping kind of distortion) so you have to listen to your ears to determine where it stops playing the output cleanly.
With this there's a bit latency but if getting the buffer setting down low enough like in my case 525 which I concider fairly usable for about anything, also gaming.