Can you bypass the JBL amp only for rear speakers to run off HU?

First post on here. I have been reading a bunch of articles on here yet having a hard time finding info on this scenario...
I have an aftermarket HU and aftermarket speakers running off my stock JBL amp (wattage appropriate speakers for the most part). However, the rear door speakers seem to need more power than the amp can provide (I've read other articles where rears sound is non-existent so not sure if this will even help). I have the PAC RP4.2-TY11 Wiring Interface to retain factory settings.
What I am looking to do is simply have the HU power the rear speakers only. I do not want to fully bypass the amp for all speakers and lose the sub (I know it isn't great/good to begin with). Will the Pioneer AVH-X391BHS HU with built in amp pushing 50x4 (what the specs say) even make the rears sound better and is it worth trying this?
Can anyone give a brief explanation on how and if this is possible without making my stock amp go nuts, sounds like when you mess with any wiring on that thing something stops working?

Thanks!

 

The easiest way to test the sound is to run speaker wires from the HU straight to the rear door speakers. Just disconnect the factory connector from the speaker. I doubt that the HU will have more output than the factory amp. You should try upgrading the speakers.

 

I have aftermarket speakers installed. I’ll try running wire from the HU and see if it can power the speakers better.