I'm actually getting an 8DB increase with my new PB13 Ultras, and they are in opposite corners. Both are individually outputting 75DB, which goes up to 83DB when I turn the other on. Very strange, as I thought theoretically the max was ~6DB increase, but i'll take it!
1 > 2 subs is more of an upgrade than 0 > 1 sub in my opinion. The DB increase is one thing, but the best part is much more even bass throughout the room. Once you go dual, you never go back.
You are getting room gain. Sometimes it's a good thing, sometimes it ain't. All depends on what freqs are being boosted.