Prime or Ultra center? The Prime center is less sensitive than their other Prime brothers (towers and bookshelfs). I know this because I once too had the Prime center used with the A850, which is basically the same receiver as the A880 sans DTSX support. In this case, the Prime center always does run hotter than the L/R mains.I have in fact been putting it on the couch at ear level, didn't even think about the couch absorbing some of the energy. Guess I'll have to get a camera tripod and redo it.
As far as the center goes, the 880 also tested 2db hot, so it must be either my setup, or Yamaha is purposely running it hot. The only reason I say that last part, reading through the 880 thread, there were a lot of people complaining about a soft center channel. Not sure how much more or less sensitive the center is, they are SVS speakers, towers and the matching center speaker. I actually prefer to place the center speaker under the tv on an angled stand, and it sits about a foot closer to me than the mains. I find this setup, at least to me, the center tends to blend in better and is less easily localized.
As for the Ultra center, SVS rates them the same sensitivity but IME - they're not either. The reason for this is because SVS seems to have underrated the sensitivity of their Towers and Bookshelfs as well, the 88db/1m is really more like 90db actually. And one last thing of note, the Ultra speakers are really 4ohm speakers, NOT 8ohms as advertised by SVS. Yamaha receivers do not do well with 4ohm loads, especially not the 880.
Here are super in depth scientific measurements of the Ultras if you want to learn more - SVS Ultra Bookshelf Speaker Review