found my answer..someone else on another forum felt the same way I did and it is R2 on IVB for me!!!Not sure I can answer your question. I barely ever used the RHY2 channel on the Mark IVb as mine was a combo and it was too bass driven. I mostly used RHY1 on a few occasions and mostly ran the lead channel. Having to share midrange and bass between RHY1 and RHY2 was a pain in the ***. Reminded me of the Mark III, RHY2 never made any sonic sense in the combo and was not a good match for the MC90 speaker. Muddy or too much bottom end. I did have a day to mess with a new 412 cab and the Mark IVB. That was the only time the RHY2 actually sounded good. I sold the Mark IVB so there was no point in backing out of the deal. I had more interest in the Royal Atlantic and the Roadster at that time.
I would personally inquire about the Mark IVA vs Mark IVB. Look for Brandon Breeze. He has both. He has a huge inventory of videos on youtube he has done over the years. He recently joined the forums.
At lease he can describe the differences in the RHY2 channels between the A and B version.