I just filled my risers with insulation. SXDT, every room is different. Just raising the sub off the floor may/may not improve things. Sub frequencies will vary as you walk around the room, even in 1ft increments. Corner placement can give you the most output, but excite all the peaks/nodes in a room. If your room has openings into other areas of the house, often placing the sub near the corner furthest from those openings can help, but there is no quarantee.
As others have mentioned, the sub crawl might help. Basically, you place the sub in your seated location, or move the chair first. Then crawl around the room as while you play something with good low bass, then just swap positions.
