Subwoofer placement

I've used subwoofers in the past but haven't had a need for one for quite a while because my speakers are sufficiently low frequency capable. Last week, though, I ordered a different set of speakers which I thought may need a small sub. I bought the Martin Logan Dynamo 500 and, in reading the locating instructions learned that they recommend placing it next to one of the main speakers if only one is being used.

Until now I've never paid much attention to placement having had them in front as well as close to the back wall near my seating area. I've never had a problem at either of these positions with directionality.

This brings me to my question; as a rule of thumb, at what upper frequency does the sub's location become apparent to the listener. I can place it in the front but, because of the room's size and layout I would rather not put it there. I realize the proof is in the listening but I'm just looking for a starting point.