I'm seriously not going to argue with someone pointing at design programs for all the answers. There is not a program out there that will tell you HOW it sounds. They're a design tool. You still have to build it.
Passive radiators are not ports.
They can operate in lieu of a port, but they are not simply ports.
They're not as "boomy" as ports, sure.
However, you now have to consider the compliance of the cone.
The usable surface area of the cone.
The mass of the cone.
You have to tune the passive radiator, like a port...you do this by adding mass to the cone.
Turning it into a mic...turns it into a mic feeding back at resonance, at any given volume. Oops. Sorry.
Duramorte, basically...the reason why we don't see passive radiator/mic elements on cabinets is that they're...too sensitive...in the wrong sense...If there is any vibration whatsoever in the cabinet, it is transferred to the microphone element.
An infinite baffle is a good candidate for a PR (though they work fine w/ smaller cabs, as long as you have space to drill for one), though I don't necessarily see how it would benefit a isobaric...jesus, does anyone actually use an isobaric?