The findings are interesting. I am very curious to see how the frequency plot would look with , and without the limiter.

With the limiter between the pre amp and the power amp, you are forcing the power amp into class a mode (if it is a class b amp). This will affect your readings. Using S.W., see how the amp measures for the 2 tests. If there is no change, then the difference is in how the speaker responds with a DC bias in the signal.

-Linc