Thanks for posting the data, though I'm not sure you can conclude much without more testing. For example,though you couldn't measure it, the likelhood is you are clipping that amp and causing compression and unwanted harmonics from the 50hz signal. In other words the SPL from the woofer is not increasing as gain exceeds rated output of the amp. If you were running a mixed frequency signal you would hear the distortion from the HF drivers as the dynamic range compressed.
Clipping and damaged voicecoils go hand in hand so be careful.
