It's because the Qms of the PR is a measure of how "peaky" the PR's resonance is. A lower Qms will result in a more damped response frmo the PR at resonance, and this influences the system response there as well because most of the output at resonance is coming from the PR itself.
Thanks for the input but why does this happen? I mean what's different about the PR with low Qms, i mean i have 2 PR's with same vas and diameter but the one with low Qts is 3 db less efficient? It has something to do with the surround i guess?!
A PR does not have a "Qts". That is a driver parameter. There is only the Q of the mechanical system, Qms. Think of it as a measure of damping, with Q being inversely proportional to damping of the mechanical motion. A lower Q has more damping, so at resonance there is less motion compared to one with a higher Q. The amount of damping is a property of the suspension (spider and surround).