Two days ago I noticed that both bass drivers have a slight distortion, or is it some kind of resonance, exclusively at 250hz.
Does anybody encountered a similar problem?

I excluded the signal source as a possible cause, or cabinet itself, crossover, cables.
I'll check the amplifier tomorrow, that's the only thing left.
For now, I've solved the problem by reducing the input level on power amp (luxman m03).
That distortion, if I can call it distortion, is virtually inaudible when listening to music, I only noticed it when I tested the speakers with those samples of frequencies, but I was shocked to hear it on both bass drivers.

Thanks in advance for any idea.
Sorry for my bad english.