Some build risers for their acoustic properties, especially on concrete floors; some even install tactile transducers (aka bass shakers) to the risers themselves for effect. I'd still want at least two subs for the frequency response smoothing possibilities across more seats as well as the volume of the room, tho I can't check your math due lack of dimensions on how much the risers subtract from the room total, when empty, of 5244 cuft. I'd love to play with a dedicated room like yours...but I'd have to build one....mebbe some day.