As far the brightness changing....I have no idea, I've never had any problems with that....you might want to turn the Dimmer off and see if that does anything...
To have the treble in the front and bass in the back - close the flap, press and hold the EQ button until the SFEQ comes up. You can press EQ to switch between different settings - FRT1 boosts the treble in the front and the bass in the back, and FRT2 boosts the treble and bass in the front, and the bass in the back. And there are also H and L settings for both of these, H meaning High and L meaning Low.