Regarding rear speakers:
When you go to a concert, do you face the rear of the auditorium or venue, so the musicians play from behind you?
I highly doubt it.
Yes, you do hear a
portion of reflected sound from the rear and the sides of the venue, however this is much lower in amplitude ( much quieter ) with an inherent lack of high frequencies, and delayed in time, as sound propagates at around 1125 ft/sec at room temperature.
Consider if you were in the close to the middle of the venue, 100 feet equidistant from the left and right speaker array, with the rear wall about 250 feet behind you. The direct sound would reach you from the speakers in about 0.09 seconds, then reflections from the rear of the venue would have traveled at least 600 feet to arrive back at you. [ (100+250) + 250 ]
The time of flight for a reflection would be about 1/2 second ( 600 / 1125 ) or 0.5333 to be more accurate.
High frequencies, with their short wavelengths will be absorbed simply through the sheer amount of people, and the thermals rising off of the packed crowd will cause the sound to be refracted upwards.
See the animations on this page, it will give you a better understanding of what refraction is:
http://www.acs.psu.edu/drussell/Demos/refract/refract.html
The middle animation is what I was talking about with the heat from the crowd, the same thing occurs at an outdoor concert, on extremely hot days or with wind gusts that can deflect the lobe of sound away from it's intended area.
Lower midrange and low frequencies have large wavelengths, and refract off of many more surfaces, with less attenuation than high frequencies.
Use the factory rear speakers for 'rear fill', upgrades are not needed here, unless of course the cone edge suspension is toast, or the coil is damaged. Turn them down in level compared to the front speakers, as they need not be very loud at all. With rear speakers producing high frequencies, the results are really not great, or 'realistic'.