So more than likely it's summing the rear input to give signal to the sub channel. That is why you are not man getting bass. Try and talk to someone close to you with forscan tool to make the rear speakers full range
It is that simple believe it or not.
Or use just the front inputs to...
If you use forscan it's not just eq on or off. You can remove the crossover on the rear. Instead of taping off at a speaker use pac audio LPHFD31 so you will have all front rear left and right adjustment. I went with a audio control lc7i pro to a 5 channel rockford and a shallow 10 rockford...
My understanding is the rear speakers have a high pass filter so not a lot of bass gets to the speakers. Front has a full range sound. Better to have a full range signal entering the amp
Need to definitely change from front to rear. Get full range sound out of the factory head unit. Thats the first thing I'd try. I have never used forscan but still experimenting with my 25. Once I start building the stereo ill send pics.