What tips are they on your etys mate?
Are they smaller or larger than the triflanges?
I've never had any problems with the etys producing frequencies that are evident in music. I'm not sure they would produce 20hz convincingly, but not many headphones do IMVHO. It's essentially a useless frequency anyhow.
And I agree that no headphone will produce 20hz or anywhere near it convinsingly as a lot of that requires the physicallity that only large amounts of moving air can do.