M3, how much are you looking to spend? Either of these subs will perform magnanimously in your 2400 cu. ft. room. Believe me, just one Plus alone performs well with plenty of output in my 3600 cu. ft. area. Now, if you'd like to consider a dual setup then perhaps dual Epik Sentinels will fit into the budget. And yes we're talking a single Ultra at this budget level which will surely push the performance envelope.
Check the sticky for multi-subs at the top of this forum.
Mitsubishi LaserVue L75-A91, Denon AVR-4810CI, Oppo BDP-83, Escient FireBall MX-111, APC H15
Speakers: L/R - ESS AMT Monitor, Def Tech ProCenter C2, UIW BP In-wall, UIW 64 In-ceiling, Dual SVS PB12-PLUS, Def Tech SuperCube I
Zone 2 (3 Areas through Niles HUB8, Niles MSU-480, Niles TS-110 Table Top Sensor & 3 x Niles IM/VC w/IR sensor)
Paradigm CS60R In-ceiling 2 Pair
Exterior -- Backyard Patio
Paradigm Stylus 250