You can find sound pressure peaks using an empty sealed box and a test tone (for example 315 Hz). Walk around the room holding the box with test tone playing moderately loud on REPEAT. When the box starts to vibrate in your hand that’s where a sound pressure peak is located. Some places where you will probably find these sound pressure peaks are room corners on the floor and ceiling, first reflection points on wall and floor, about a foot directly above speakers, behind and in middle of speakers at rear wall, on floor in front of and in between speakers. Just placing a vase or bottle on top of speakers should be audible although it may hurt the sound if not pre-designed, I.e., just an arbitrary vase/bottle.