The $64,000 question! This is so difficult to answer with any certainty. It is so dependent upon the particular circumstances of your listening room. There have been some attempts to give rules of placement, such as the rule of thirds. This however in practice is just so limiting. Very few rooms are of a regular shape, and added to that are the different acoustic properties of the walls, the floor, the windows and of openings into other rooms, that blanket rules are largely wishful thinking.

Some clear pointers are that:
1. Putting the speakers closer to walls and corners will give you more bass, and quite possibly too much bass
2. Location the listening seat will give you more bass, and maybe too much
3. Listening dead center in the room will likely give you a big null in the bass at some frequencies and result in uneven reproduction of bass notes.

The simple answer is that there is no simple solution. Start by placing the speakers about 10’ apart, listen around 12’ away, keep the speakers around 3’ from rear and side walls, listen around 2/3rds of the way down the room, then just keep experimenting. Listening to some music with good, consistent bass that explores the full range of the bass frequencies, and then keep moving the speakers inch by inch and listening for the most even reproduction of each bass note. It can be a long involved process, but is worth it in the long term.