Depending on the room modes, do a bit of research and you'll find 3 subs is a big improvement over two and two over one.
Good answer here: https://mehlau.net/audio/multisub_geddes/
AI summary:
# Multi-Subwoofer Setup: The Geddes Method
This technical document explains the acoustical principles underlying multi-subwoofer systems and presents Earl Geddes' evidence-based calibration methodology.
**Acoustic Foundation**: In enclosed rooms, sound reflections create "modes"—frequencies where standing waves amplify or cancel. The Schröder frequency (150–300 Hz in typical rooms) marks a critical threshold. Above it, abundant modes and human hearing's directional capabilities mitigate problems. Below it, sparse modes create problematic peaks and dips, causing "booming" or absent bass. Since humans cannot localize frequencies below 80 Hz, multiple subwoofers distributed throughout the room can smooth this response without audible source indication.
**Statistical Principle**: Geddes' key insight treats bass reproduction statistically: each additional independent subwoofer reduces frequency/spatial variance by approximately 1/n (where n = number of subs). This means the second subwoofer yields significant improvement, the third offers moderate gains, and a fourth shows negligible returns. Critically, subwoofers must be *spatially separated*; proximity creates correlation that diminishes benefits.
**Practical Setup**: Only three well-placed subwoofers are needed: one near the mains (corner), one mid-wall (not corner), one flexible placement. They require only basic controls (level, crossover, phase) and modest power—10" drivers suffice.
**Calibration**: Using measurement equipment, systematically optimize each subwoofer sequentially, adjusting gain/phase/crossover while spatially averaging response near the listening area. The result should achieve approximately ±2–3 dB spatial uniformity across the bass region.
