The added function the PMA450 introduced was 3D (known as Itelliaudio) . It is possible to set Comm1 and Comm2 so they sound like they come from different sides of the space.
See the product page for more information.
http://www.ps-engineering.com/PMA450A.shtml
At first, our club thought it would be worth having. Turns out, it's one feature that is hardly used. If one of our members sets it up and uses it, the next member often turns it off.
What the PMA450 does excel at is ease of distributing the music stream. It accepts 3 different streams (one BT, 2 wired), and can easily route the streams to the different headset stations. So if the pilot wants to listen to classic rock, the right front seat nothing, and the kids in the back want country/western gansta rodeo rap, you can set the distribution so all hear the music they want to hear. And it does not interfere with intercom.