It simulates the physical feel that in real life bass gives. We've known for a while that we "hear" bass as much through our core (chest or back dependent on the way you are facing) as we do through our ears. That's the biggest reason that nobody ever agrees on what neutral bass is with headphones, because everybody agrees that headphones need more bass than measures exactly neutral, we just never agree how much extra bass headphones need. But even if you got that amoutn exactly right, your brain sort of knows something artificial is going on, because it knows that you should be feeling anything below 200Hz in your core.
No, it isn't a bass boost for your headphones. I actually have it running through an entirely different DAC than my headphones.
If you see this diagram, "digital out" is what I have feeding the MiMBy, which in turn feeds the TorpedoIII, which goes to the HD800. AudioEngineD1 is a USB output that goes to the D1, which then feeds Subpac.
So, as you can see the headphone and subpac don't directly interact. I actually had to switch to minimum phase in Sonarworks, because linear phase meant that they were out of sync by about a thousandth of a second. Not enough to hear them as being out of sync, but enough to make the bass seem a bit "wooly" like how it sounds out of something like a M50X that doesn't have great phase alignment in the bass between L&R.
But yeah, it allows you to "feel" the bass like you would in real life. Somehow your brain then interprets this and makes it sound louder to you. It actually sounds like you're turning on a bass boost circuit when you turn subpac on, even though it doesn't actually produce any dicernible auditory sound. But since you aren't pumping extra bass into your ears, your ears retain their ability to hear fine detail, like they do when listening to a bright headphone. So you get the best of both worlds. It also, to me, seems to expand the soundstage a bit, somehow. The only explanation I can think of is that it expands the portion of my body that is "receiving" sound and thus seems more expansive.
I actually wear it backwards when sitting down, ie on my chest. If I wear it on my back, my brain actually flips the soundstage and I hear things as coming from behind me. This effect is very pronounced with non-angled headphones. With the HD800, it's kind of a weird effect having it on your back, it's like the bass is behind you but the rest of the band is in front of you. But it's perfect to just flip it around and put it on your chest.
![DT1990-ChSd-1024x555[1].png](https://cdn.head-fi.org/a/12230656.png)
