+1 for multiple subs. If your mains go very low, like your 804 D3's, you're stuck with trying to balance two issues: imaging (driven by speaker placement) and room modes (dictated by speaker placement). You would be incredibly lucky if your solution to these two problems happened at the exact same speaker placement!
By separating the two, you get to put your mains in the ideal place for imaging, but get to move the low frequency sources (the subs) wherever is ideal to minimize problems with room modes. Conquering modes is where "the more, the better" mantra with subs comes in.
To really get this separation, you need to high-pass the main speakers, and low-pass the subs, so the sound level and phase are (mostly) matched. Using an active crossover and/or DSP are ways to achieve this.