@Steve-Mohican I'm not an expert in reading phase diagrams, but this looks alright to me, the phase is supposed to wrap around the frequency split points and it looks like a somewhat linear phase to me.

No.

Can you set the library to use float numbers as signal? Usually every library allows to define the floating point type used for its signal (eg. Faust is using FAUST_FLOT which you can define to either float (reasonable) or double ("yeah, 64bit signal precision for the ultimate transparent sound").

If you need to feed it double numbers then it will become very annoying because you can't just cast the pointers to double pointers because the original source are still float numbers and you would have to introduce a working buffer that uses double numbers, copy the float numbers over, process the working buffer with your library and reverse all those steps to copy it back to the signal buffer.