Most useful tweaks that are sensible and really make a noticeable improvement

I wonder why no one mentions Master Setting speakers.  I went to a Denver audio store and they demonstrated master set.  He had his speaker location taped on the floor so after moving them out of position they would go back into sync.  He demonstrated how everything sounds better when you can not detect either right or left speakers.  When he moved the right speaker about a foot out of position, we suddenly had to raise our voices in order to hear each other.  The moment he moved the right position, we were able speak at normal conversational volume levels.  I also noticed how my shoulders dropped.  Everything sounded relaxing and the sound came from between the speakers to create a really nice sounding sound stage.  However, when he demonstrated the REL S3 subwoofer, I was not able to detect a great deal of difference.  I think this is because REL designs their subs to act more like woofers than subwoofers to allow them to play down slightly lower.  This is probably why they blend so well.  However if the specs for the S3 say they play down to 21 Hz at -6 db, they are most likely playing down to maybe 30 Hz.  The problem is if a recording contains frequencies that play down to 14 to 20 Hz, you are not going to hear them.  The trick is finding a subwoofer that doesn't pound you out of the room.