About a week or two ago, I saw a discussion on Hacker News about How Surround Sound for Headphones Works. Linked within the article, as well as the source item's page, was a free tool for OSX called "Hajo's Headphone Enhancer". I downloaded and installed it, without giving it much more thought. However, I was working on my Macbook today, and thought I'd pop in some headphones and see just what kind of difference, if any, I would notice.
As tersely as I can put it - WOW! Alternating between different settings, on songs I've been listening to for ages was absolutely stunning. Rather than feeling like I was listening to a music album, it felts as if I were a fly in the studio during the recording sessions.
I used it for all of 45 seconds when I decided that this was absolutely worth the 10$ to register the software, and avoid any nagware screen after the free trial period. You can keep running it, as the author says on the page that it is free, however, as with all effective nagware, a prompt/reminder to register will appear every so often - every 30 minutes if I am remembering correctly.
As you can see by the options for audio/ambiance styles, it's not as elaborate a configuration as Boom 2, which did sound quite nice when I tested it with the built-in speakers (I did not try "Boom 2" with headphones before my trial period lapsed). However, the counter argument of "it's not as elaborate a configuration" makes it quite simple to pick a setting, give it the thumbs up, and rock out! But maybe that's just the knob fiddler in me trying to set different things within Boom 2 in a "let's see what happens now" attitude!
tl;dr: If you run OSX and use headphones, get "Hajo's Headphone Enhancer" and pay the unquestionably cheap but totally worth it 10$ for registration!