From the white paper I also find interesting the analysis they did on music for the crest factor and their amplification choices based on that:
One important advantage of an active loudspeaker is that the amplifiers for each driver may be specially designed according to the specific signal characteristics each will see. To gain a better insight into the requirements, KEF R&D performed a statistical analysis on an extensive library of more than 40,000 pieces of real music. This study provided new data on the spectral amplitude and crest factor in different frequency bands. Crest factor is the ratio between the peak and average amplitude of a signal. Low and midrange frequency content tends to be continuous with a low crest factor. As frequency increases the crest factor raises considerably.
Figure 25 shows an example calculation of the crest factor across five midrange and high frequency bands for a sample of 1800 music files.
Figure 25. Calculation of mean crest factor by frequency band on statistical sample of music
For an active loudspeaker, this means that the LF amplifier must be capable of providing continuous power output. A HF amplifier, however, needs to provide only very low levels of continuous output, but still be capable of outputting extremely high instantaneous power peaks. KEF have developed a sophisticated software simulation tool that mimics the end-to-end signal path in the loudspeaker, including details such as the DSP processing, the frequency response of the drivers and the complex load that the drivers present to the amplifiers. Using this tool the exact power requirements for each section can be calculated for a range of different musical genres. This provides detailed power statistics for each driver that can then be used in the design and specification of the corresponding amplifiers. The result of this analysis on the LS60 Wireless was that a 500W amplifier was required for the LF portion, 100W for the MF and 100W for the HF. Based on the bandwidth and signal statistics a hybrid amplifier class arrangement was selected. The LF section of the system demands high amplifier power to deliver clean bass output. The MF section also requires high continuous power output on certain demanding tracks. Along with the constraint of housing the electronics in a separate enclosure at the bottom of the cabinet, this requires thermal efficiency to be a priority and hence Class D amplifiers were selected for these sections. Class D amplifiers are highly efficient due to their high-speed transistor switching design and their sound quality can be at a level required for high-fidelity reproduction. The HF section has very different power statistics. Only low continuous output is required but the amplifier must have the ability to deliver sudden large peaks. High bandwidth is also an important consideration for true High-Res support. For this reason a class AB amplifier was selected. Class AB amplifiers have a much lower efficiency than Class D, but the thermal power generated is reasonably low under these signal conditions. Class AB has the added advantage of wide HF bandwidth and of not requiring an output filter. The reason behind the high-power specification for both MF and HF amplifiers, especially the HF one, is due mainly to the crest factor of music at the intended frequency passband. Ensuring the amplifiers have the rail voltage available to supply the power to amplify a sudden sharp peak in the signal is crucial to avoid harmonic distortion. The KEF Class AB design was revised from LS50 Wireless II to further reduce distortion.
Makes the most sense. 4" Uni-Q is interesting though!
