Well... I think they're very close... but I think the iMac Pro speakers have a bit deeper bass to them, but they also seem to distort at higher volumes a little easier. So...
in my opinion...
iMac Pro Speakers: More bass, but also more distortion
Powermac Pro Speakers: Slightly less bass, but less distortion at higher volumes.
Is there anyway to run both sets at once? Using a splitter or some thing?