Speaker Placement or Room

Speakers are Vandersteen 3A's. I've got them set up according to the placement instructions that came with the manual. In general they sound really good.

I've just taken some readings with my Radio Shack spl meter and the Rives test disc, using the frequencies that are corrected for that device. I know no room is perfect and a flat response will not happen, but for the most part everything is "relatively flat". However, I'm getting a huge dip between 80hz & 100hz. 80hz is -23db, 100hz is -12 db and 125hz is -15db. 160hz is back to flat.

Is this an issue of speaker placement or is it the room? If it's the room, where do I begin to minimize these dips?

Thanks.