While I can't solve your apparent problem of room size and speaker placement, I can offer one suggestion that will somewhat "tame" the output of the rear bass port. Years ago, I tried the following "tweaker's" suggestion, and found that it worked quite well to both reduce and tighten the output of the rear port:
Buy several boxes of plastic drinking straws -- ones that have fairly large bores (such as you'd use with a milkshake), not the little "stirrer" straws. Cut the straws so the length of each straw matches the depth of the port. Then assemble enough straws in a bundle to tightly fit the port, without crushing any of the straws (it helps to use a rubber band to assemble the bundle). Make a bundle of straws for both speakers. Insert the bundle of straws into the port's opening, and push the bundle into the port until the mouths of the straws are flush with the port's opening.
Filling the port with the sections of drinking straw adds resistance to the air being pushed out of the port, thereby reducing the output, and also tends to flatten the response curve.
If this solution doesn't help, you have only spent the cost of two boxes of drinking straws, and the "tweak" is easily reversible by removing the straws from the ports.
