I suggest taking your basic standard equilateral triangle placement and rotating it in the horizontal plane somewhat, such that your setup ends up somewhere in between "square" and "diagonal". This should smoothe the bass relative to a symmetrical setup. If your room is rectangular rather than square, then you could try a true diagonal setup.
The more nearfield you listen, the less the relative influence of the room. So you might try a nearfield variation of this quasi-diagonal setup, if you have the freedom to do so.
Credit to Audiogon member Clio09 for educating me on quasi-diagonal nearfield setups; I wouldn't have thought to try what he's doing.
Duke
dealer/manufcturer