You forgot a step that many here don't seem to talk about but could be absolutely crucial to setup, especially with those using more than one sub that are different brands. After you have done your gain matching individually, I would stack or set side by side the subs you are using and use a Y connector to hook both up to your AVR. Now run them both together and make sure that they are additive (level goes up!). This ensures that the acoustic polarity of each sub is the same (sometimes they won't be!!) and that they are not going to actually work against each other. No one wants to buy new subs only to find out they are working against each other in the room, but this does happen. If for any reason you actually do run into different acoustic polarities (it will be pretty obvious if this is the case) you will have to switch the wires on the sub driver for one of them. Contact the vendor who makes you sub for more info on this if you run into it.
Good suggestion, Kris.