Three-way with shared cabinet for woofer and mid not behaving as expected.

Hello,

I've decided to skip building boxes for a threeway speaker and repurpose some old cabinets.
I've swapped the drivers but there is a small problem regarding the midrange. The original one had a plastic cup behind it, and I tried putting the new mid in it but it didn't fit..
For the time being, I just left out the plastic cup and mounted the mid straight into the enclosure, so now it shares the same space with the woofer.

I expected that the midrange would be suffering from the bass, that it would move so excessively that I could only listen at very low volumes.
But it doesn't.
How could this happen?
 
It will almost certainly increase the midband distortion. Although such an arrangement was to be found in the dim and distant past, I don't recommend you adopt it!

The midrange cone suspension in your case may be stiff enough to hold its own against the pummeling from the back pressure of the woofer, but that doesn't mean the cone is vibrating as the applied midrange signal intended!