We just upgraded the OEM sub on a DSP system because of how bad it sounded.

Those 6"s play 80 hertz and down, but they have a mechanical resonance of 80 Hz too, so they roll off at 12dB/octave at their crossover point. This means that they are 12dB quieter at 40 Hertz than at 80.

The 3mm thick ABS panel in front of them doesn't help either.