In my large room, with 7 channels, I have the main L/R mounted on top of 15" subs. This, in effect gives me 4-way mains. Works great for HT and multichannel music.
But in my smaller room, that has a turntable setup I only use one sub, which is in between the two speakers.
In the first room that I ever used a sub I had the sub in the front corner. That room had bad standing waves, due to a steep vaulted ceiling, that topped out at 15' and the wall then dropped down to the 8' level.
So it really comes down to what the room needs.
But in my smaller room, that has a turntable setup I only use one sub, which is in between the two speakers.
In the first room that I ever used a sub I had the sub in the front corner. That room had bad standing waves, due to a steep vaulted ceiling, that topped out at 15' and the wall then dropped down to the 8' level.
So it really comes down to what the room needs.