I’ve measured and I have seen measurements that it is 120 dB.
Why? Now the filters vary from 192 dB to 300 dB.
And truncated early transients and constant ringing… 
But not unconstrained factors. Since I’m doing minimum 256x with 64-bits. That would produce pretty horribly big files…
It is property of the filter in question. Use some other filter, like sinc-Mx for higher attenuations (~ 240 dB).