Gonk and others:
Last night I tested the "Super speedway" dvd disc on DTS straight 5.1,DTS Neo:6 and then DTS + PLIIx, at -12dbs.
You know what, my preference priority is exactly the same listed above. Appears to me that DTS straight 5.1 produced the most distingtive sound separation and positioning and the bass was stronger and tighter. DTS Neo:6 came 2nd, as the sound was not as focused and the bass was weaken a bit. The DTS + PLIIx had the weakest bass among these 3 modes.
In my opinion, we would get the best sound quality and bass from the mode that the disc offers. Extra surround processing somehow would degrade the sound quality a bit. NEO:6 and PLIIx are meant for 2-channel source.

Kwok