OK, here's my take on the problem:
1. "hi-pass" filter on the amp cuts the low frequency to the doors and rear. THE OPPOSITE of what you want. Turn it off.
2. Sending full range audio to the dash speakers (even though they are technically full range speakers) is a mistake. The small speakers have a hard time reproducing treble accurately while also handling large-excursion low frequency signals. Install a crossover (just a properly chosen capacitor in line is fine).
3. The overall gain of the door and rears channel (output of the HU - loss in any "speaker to line level" conversion + gain of the power amp) needs to be set carefully. This is probably the main reason for the imbalance. The way I would do this is to pick an audio tone which is in the range of both dash and door/rear speakers (or a 'pink' noise source). Play the tone, and set the amp gain to achieve equal SPL when fader is full front and full rear.
The quick and dirty alternative to all of this is to play a song you are very familiar with, and adjust the front/rear fader on the HU until it sounds right to you.
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