If the options are adding another matching sub or new surrounds, I'll take the sub all day. More immediate gratification.
But I would look into a new complete front soundstage. I think your money would be better spent there.
HD-DVD: 55 / Blu-ray: 97
newest: Brotherhood of the Wolf / Kingsmen, How to Train your Dragon
AVR: Marantz SR6006
F/C/R: Polk RTi A9's / CSi A6 / Speakercraft AIMWIDE 3 / Sub: SVS PB12-NSD
SonyXBR-75X940D Samsung BD-P2550, Toshiba HD-A30, Nvidia Shield TV (2015), Xbox360 Elite, PS3, WDTV Live