Just saw this thread. Instead of trying to put more speakers in-between or behind and using HT modes to artificially create a larger soundstage/ambience (such as HALL mode, etc.), I would try putting some diffusor panels on the front wall in between the left/right speakers. Try looking at the GIK Acoustics Q7d diffusor. Two or three of these mounted up on the front wall vertically about the listening level should make the room sound a lot larger.
Beyond acoustics, I found that quality electronics can affect soundstage. Look towards upgrading power cords connectors, fuses, interconnect connectors. Go for rhodium plated Furutech all the way through (including the fuses). This should significantly improve the resolution and soundstage depth of your audio.
Trying to use surround modes such as HALL or ORCHESTRA can make it sound like your in a large venue or echo chamber, but it lessens the attack and impact of sounds, which is important in my opinion.