Mukund,
It seems like PLII/NEO:6 would also have a better "room filling" sound to them.
Filling a room with sound is not the intent of matrix decoders like PLII and Neo:6. If that's what you're after, then you should stick to 5/7-channel stereo.
The 5/7-channel stereo mode basically copies the stereo signal to each pair of speakers (fronts, sides, rears) and sends a summed L+R signal to the centre speaker. Since vocals are heard from all 5 or 7 speakers and the same stereo information is coming from every direction, there's no attempt to create a soundstage or any specific imaging. This is the mode to use if you want to fill the room with sound.
PLII has a different goal, primarily to stabilize the soundstage for 2-channel material. Stereo material contains correlated (in-phase mono) sounds and uncorrelated (out of phase) sounds. Correlated mono info images in the centre of the front soundstage. This is the information that PLII extracts and moves to the centre speaker. This is where those sounds would image anyway, except now they're locked to the middle of the front soundstage and can't drift to the nearest speaker like a phantom image does when you move.
Uncorrelated sounds tend not to image in the front soundstage. If you've ever heard an out of phase signal (most test CDs have good examples) you'll notice that it sounds like it's coming from all around you. On good recordings, these are the sounds that image outside your front speakers. These are also the sounds that PLII moves to the surround speakers. If the recording didn't have much uncorrelated ambient information, then you won't get much in the surrounds. The point is to honor the intent of the recording, not arbitrarily fill the room with sound. Again, completely different goal from the 5/7-channel stereo mode.
If the music and TV programs you listen to sound better to you in 5-channel stereo, then that's the mode you should use. It will consistently give you the "room filling" sound you are looking for and not be dependent on program material.
This is even after (for PLII M) adjusting Panorama, Dimension and Center. They don't seem to have any appreciable impact.
That's weird; adjusting those parameters has always had an immediate and noticeable effect. If you want more sound in the surrounds, tilt the Dimension control towards the rear setting and turn Panorama ON.
Best,
Sanjay