To get a better look at the maxima and minima of, e.g., the 2,1,0 mode at , use a tetrahedron plot. The relevant quantity when it comes to placing the loudspeakers is the amplitude of the standing pressure wave. A sound source will excite an eigenmode most if it's put in one of the mode's pressure antinodes. Conversely, with the source in a pressure node, the eigenmode will remain silent. Check Tetrahedron as Plot type on the General page.
All modes have local maxima in the corners of the room.
Speakers placed there will therefore excite all eigenfrequencies. If you change the Keep type to min , you will notice that most modes have a pressure node in the center of the room. Finally, add a plot of the pressure distribution on the walls and furniture. Check Surface under Plot type on the General page. To see not only the outside of the walls, but also the inside and the tetrahedron plot you have to remove the obscuring boundaries.