Merlin TSM makes rattling noise at low and loud

I am a newbie in this area. I have a pair of Merlin Master TSM MXM. And I have been enjoying them. Today I bumped up power of the amp (iNova) to about 80% (max is 80watt), and one of the speaker sends rattling noise when playing very low frequency. Turning down the power to about 60% and it is gone. I switch cable (Morrow SP4 and a cheap-no-name-one)/speaker left-right combo. That particular speaker makes louder rattling noise. The other also makes some but less.

So, my guess is the front rubber layer of the woofer cannot hold itself in that kind of playing setting. Is it expected? I did try on SWAN 2.1P with the same setting. I heard similar noise, but far less.

Still, I'm enjoying the music. Among 100+ songs I played there is only one has extreme high and low which revealed the problem.