Sub placement

It's true...bachelors often put their subs in the middle of the room.

Wolf_garcia, regarding your comment:

The "put the sub in your listening spot" suggestion is ridiculous (but amusing) so ignore that one

You are simple incorrect. Placing the sub in the listening position and then walking/crawling across the room to find the place with the most even bass response, helps one find the place in his/her room that beside having an even bass response it is also the acoustic equivalent of the listing position, i.e. the interference pattern of the low waves in that place is very similar to the one in the listening position. The method is not ridiculous at all, in fact it is based on simple 3-dimentional Euclidian geometry.

Placing sub under your listening chair with woofer cone up will create an extra stimulation of your hips groins and genitalias. Your wife/date/girlfriend may like that idea
:-)

Assuming your two subwoofers are getting separate left and right channel signals treat them like extensions of your main speakers by placing them next to, but forward of, your main speakers.

The listening position method is meant for single sub placement and it can be very successful. Unfortunately, those locations can be obtrusive to multi use rooms. Still, if you have the time the procedure can give you another character aspect of your room.

If your subs lack digital signal processing then a product such as DSpeaker may be of great benefit to better integrate your subwoofers into your room and system.

I use 2 M&K KX10 8'' subs with a pair of Lipinski L707 monitors. I place them just behind and to the outside of each speaker and they sound as one. You need to keep the subs away from the amp and rack to keep vibration to a minimum. Best of luck.