In Cubase 8x over Cubase 7x only changes have been made to the filter slope. In C7 both high and low pre filters were set to a fixed slope of 24dB/8ve. In C8 this has been changed to an individually selectable slope of 6,12,24,36,& 48 dB and the default value is now 12dB. This is an overall improvement, however for migrated projects, the old value of 24dB probably wasn’t saved (because it wasn’t an option) so the default value of 12 is probably substituted which is going to impact the sound.
Would have been better if one could set a Pref for the default value and/or leave the default set at 24 - might have prevented the current quandary.
As it stands, I don’t know if this could be corrected for all audio tracks in the PLE ?