My two cents....place them at the same distance to your listening position as the speakers....with two full range speakers you have a lot of drivers covering the 20 to 90 Hz range (even after filters)
Therefore place one on the left of the left speaker and slightly in front along the wall (maintaining equal distance to the listening position as the two speakers). The second you can do the opposite.....right of the right speaker, slightly in front and along the right wall.
As Duke says, there are two schools of thought......I have tried both (but only with one sub);
My impressions are:
1) If you seek smoothest sound by locating the sub in any old position there will be inteference between the sub and speakers on the primary signal, as the sound waves are not aligned at all frequencies. This makes the room reverb better but it destroys the clarity of the primary signal. The bass sound loses impact and is slightly disconnected from the music. ( A solution for this is to use the THX rule of cutting your main speakers at 80 Hz and using the sub for signals below that....again I find this disconnects the bass from the rest of the sound as there is plenty of energy in the 50 to 80 Hz region despite the filtering)
2) If you seek alignment of the sound with the main speakers by placing the subs in front of the listener at the same distance as the main speakers then the sound will be tighter and more integrated for the primary signal....to me this sounds best, despite room modes being noticably more excited in this configuration ( I use a PEQ to take the edge of room modes by slightly reducing the sub signal at the bumps)
The above are my observations after much experimentation. I can't say that I can locate the sub except when I am within a few feet and it is obvious where the high pressure comes from, however, I do sense when the primary bass sound waves are aligned and integrated at the listening position. The sound becomes tighter and cleaner and the "room sound" is also correct(but may be more intrusive).
By "room sound" I mean that the reverberation in the bass now matches the rest of the audio reverberation signals...I suspect our brains are able to distinguish primary from reverb signals and one can sense when the sound is aligned for the primary signal (aligned = coming from same direction) ....this may explain how we can so clearly hear someone at our table in a large dining hall with tons of people talking,