We have had our Sleep Number bed for over 15 years and still LOVE it! We are big people and were going through mattresses quickly. With this we adjust it as needed and I sleep so well. Now pillows are another matter! We finally broke down and spent $125 at Brookstone. I got one for my husband years ago and finally got my own recently after trying several cheaper pillows that all flattened to quickly and didn't give the support I need. It has made a world of difference. I wake up refreshed without my back hurting. I am a side sleeper and my pillow no longer collapses on me but gives me the firm but soft support I need.
You may need to play with your bed adjustment. I need mine up at 90 where when I first got it I started at 35. I find when I sleep and my body gets hot it is time to redo the air.
Karen - we recently moved and had to take the bed apart. When we were putting it together we realized LAST time we put it together we put the bottom together wrong and had the same middle dip you talk about. It was squeaking, noisey and sagging. Now it is back to where it should be, nice and flat and quiet! No more sagging. You may want to try taking it apart and redoing it.
Hope this helps and you get a good nights sleep!