It is OK for the sub to be near the corner, but your mains should not be. Bring them in closer, pull them away from the wall behind them at least a foot. The rule of thumb is, they should be about as far apart as you sit from them, however if you are 15' from them, that may not be reasonable. For music, that rule is not a firm rule and it depends on the room, so experiment with placement.
If you are sitting right up against the wall, this will also degrade your sound. Consider hanging something soft on the wall immediately behind you; aka sound panels, but a rug, curtain, etc... should at least help too.