Setting all the speakers to 0dB in that sense isn't too important. You need to set the front left to 0, turn up the master until you register 75dB on the front left, and then in turn adjust each of the other speakers individually to 75dB. Does that make sense? Basically you are setting the relative offsets for each speaker with respect to the front left.
Also (at least on my reciever), the settings for equalizing the speakers are stored independently from any individual speaker ajdustment I make for a given playback format. So, once in the setup menu I've equalized everything, if I then start to playback something in Dolby Digital, all my speakers are listed as being at 0dB and I can turn up the rears if I want. But I know that at least I'm adjusting from an even starting point.
