Typically maximum sound pressure level (SPL) figures are quite simply specified as a number. This means nothing unless the measurement conditions are known: frequency range, test signal, measurement type, distance of the microphone to the loudspeaker, duration of the signal, location of the loudspeaker, etc. As an example, here is the specification for the O 410:

Maximum sound pressure level in half space at 3% THD at 1m averaged between 100 Hz and 6 kHz = 120.0 dB.

Changing any of the measurement conditions will give a different value. To measure the maximum SPL, a sine wave is played into the loudspeaker and it is increased until the distortion reaches some value, say 1% or 3%, and then the SPL is noted. Sweeping the sine wave’s frequency allows a graph to be plotted or an average to be calculated. Alternatively a broadband signal can be used, for example, pink noise, IEC-weighted noise, or “program material” (whatever that means!).

As with most aspects of acoustics, maximum SPL is frequency-dependent. Lower frequencies are harder to reproduce at high levels as the displacement of the drivers must be high (four times the displacement required with each halving of frequency). This is seen as a drop in maximum SPL with reduced frequency.

O 410 Harmonic Distortion at 95 dB SPL
(Red: 2nd harmonic, Green: 3rd harmonic)

The loss of headroom at low frequencies can be made up by adding a subwoofer to the system:

KH 870 Maximum SPL at 1 m  
(Red: 3% THD, Blue: 1% THD)

O 410 with KH 870 Maximum SPL at
1 m (Red: 3% THD, Blue: 1% THD)

Note these measurements are for a subwoofer located in half-space. Subwoofers are typically located in quarter-space and so the subwoofer part of the curves (below about 100 Hz) would be 6dB higher.

Adding a subwoofer also brings a decrease in the low-frequency cut-off of the system, reduces harmonic and intermodulation distortions (even in the midrange), and changes to the group delay.
Adding a subwoofer to a smaller loudspeaker gives a more dramatic improvement to the maximum SPL.

O 300 Maximum SPL at 1 m
(Red: 3% THD, Blue: 1% THD)

O 300 with KH 810 Maximum SPL at
1 m (Red: 3% THD, Blue: 1% THD)