when i place q3 on a mono insert in cubase 9.5 its doesn't seem to want to process as a mono plugin . ie when i put a brick wall low cut on and sweep it up i can still hear the frequency i am trying to cut.
check this: www.fabfilter.com/forum/3586/pro-q3-not-processing-correctly-in-cubase
There is indeed an issue with the VST2 version of Pro-Q 3 in Cubase. Something is going wrong with the channel assignment, and the input signal is sent directly to the output signal in parallel with the processed, eq'd signal. Therefore you will always have a mix of the eq'd signal and the un-eq'd signal, and some processing can be barely audible.
We suggest you use the VST3 version of Pro-Q 3 instead. We have addressed this issue with Steinberg, but unfortunately they have not made any changes.