Crossovers
Small and large loudspeakers should be crossed to subwoofers based on measured capability and room behaviour rather than badge size.
LFE vs redirected bass
The LFE channel is not the same thing as bass redirected from speakers below their crossover.
Independent subwoofer channels
Multiple processor outputs allow different level, delay, EQ and routing strategies for each subwoofer or array.
WaveForming / ART / Bass Control
Advanced systems move beyond basic EQ and use multiple subwoofers to control how low-frequency energy behaves throughout the room.