The surrounds should technically be levelled to the same SPL as the front speakers. All the speakers in fact should be levelled to the same reference SPL. The studios where the films soundtracks are mixed will all adhere to this same standard and the ideal of home theatre is to try recreate what you'd hear in a cinema. A cinema's speakers are calibrated and levelled to try mirror the standards used to setup the studios where the soudtracks are mixed. I have heard it said that theatres sometimes set the rearmost speakers slightly higher than those toward the front of the theatre though. This would in fact be the opposite to what you want to do, but I doubt your room to be as large as that associated with most movie theatres or your listening position to be as far from the rears as someone sat in the middle of a theatre would be?
Th ideal is to set all speakers as being the same SPL as measured from your listening position. Anything other than this is done in accordance with your own personal preferences. The ideal gives you the closest you can get to what the person who mixed a soundtrack heard because the manner in which the speakers are levelled is standardisd right the way across the film industry.
Maybe the surrounds are too localised as opposed to being too loud? The distances you mention are far enough away, but where are the surrounds located in relation to where you are sat? Is the floor level a 5.1 or 7.1 layout?