First thing, I don’t think the system is overkill for the room. I think you have some excellent electronics and speakers to enjoy!! The two downsides I can see are the quality of your subs and the room.
The SVS subs at $499 each are probably going to be a weak spot when matched with your higher end Revel speakers. The PB1000 is actually a pretty large cabinet, but only weighs 46 lbs. This tells me that they are using thinner wood for cabinet construction that isn’t well braced. I have heard other subwoofers that just rattle/vibrate the cabinet and it takes away from the sound quality. Also, at this price level, you are not getting a really good amp or woofer either. Don’t get me wrong, this is probably a very good value sub, but if you are listening at the $6k Revel level, you can do better! The boomy character of the sub can be from the cabinet construction, woofer quality or amp quality. Maybe look at getting a couple used JL Audio or Rel Acoustic subs. The JL e110 is a smaller cabinet than the SVS, but weighs more at 52 lbs. This means better cabinet stiffness, better woofer and better amp. There are many subs up for sale in your $5k price range -- JL Audio, Rel Acoustics, etc.
The other downside is your room. I have read that perfect square rooms are the worst scenario for acoustics. Your room is almost perfect square. You generally want a more rectangle room, but not a perfect rectangle, so your 20x28 is a good plan. If you have your speakers placed in the corners (which is the same as my room), you probably have standing waves bouncing between the corners. Check out GIK Acoustics website. My initial thoughts would be to stack two 24"x48" bass traps in each of the front corners (or the thickest acoustic panel you have space for). You can also put some bass traps with the FlexRange membrane in the rear corners. The FlexRange membrane will absorb more bass frequencies and NOT absorb the mids/highs -- used if you want to keep the high frequency energy in the room. I’ve found that there is a fine line where there is too much sound absorption. If you have carpeted floor, you can hit this point easily.
If you still feel you have problems with imaging, maybe start looking at upgrading power cords, interconnects, fuses.
