Thank you very much @alain and @Raymundo.Cassani for your in-depth answers!
Appreciate your time to write detailed answers 
We've collectively determined that the main issue has to do with filtering as we've had a similar experience in the past with Brainstorm filtering.
Quick answers to your questions;
- ActiRead highpass filter type; 1st order. ActiRead lowpass filter type; 2nd order Bessel.
We asked the BioSemi team for a frequency response but haven't received this information. - The filter was applied before averaging epochs.
- I do not think that there is a baseline definition in ActiRead, as such I don't think baseline correction exists.
- ABR stimulus was 2-1-2 tone burst at 4 kHz.