Surround back speaker. Same as front center?

The center rear surround speaker(s) is a separate channel from the center front speaker on Dolby & DTS ex/es. On source material that does not originally have the 7 channel configuration the center rear channels usually are summed from the 2 side channels assuming a 7.1 processor. On 5.1 systems the rear channel info is summed to the side channels. Original specification called for all 5 (or 6 or 7) speakers to be the same, usually with integration into stereo systems compromises are made.

As I understand it, the center speaker is important to focus/image the speech or dialogue audio in the movie to match the location in the image viewed. It is important for this to blend properly with the left and right channel speakers so should be "voiced" accordingly (probably should be same manufacturer and with usage for that purpose). The rear/side channels provide "ambient" sounds and should not be so noticeable as to draw attention to themselves when properly set up. Hope this is helpful.