VU meter / lowpass filter
With lots of high power amplification & expensive drivers, I think it's a good idea to monitor the system power levels. This will be done using a VU meter from Qkits incorporated into Samson's Marchand XM1 active lowpass filter.

Here are the technical specs :

K4306: Stereo VU Meter kit

2 X 15 LED's
Scale : -23dB to +5dB, 15 LED's per channel.
dB-linear division from -4 to +5dB (1dB per LED).
Systematically growing steps below -4dB.
Accuracy : 0.5dB (1 KHz).
Frequency range : 20 Hz to 30 KHz (-3dB).
Three different LED colors : green, orange and red.
Adjustable sensitivity: 150mV to 5Vrms Low input, 1.5V to 50Vrms High Input
Input inpedance : 47K Low input / 470K High input
POWER SUPPLY: 10 to 15VDC / 300mA max.
PCB DIMENSIONS: 150 x 57 mm
A couple of modifications were made to the input coupling capacitors to lower the lower -3dB to 10Hz, and in the trimpots which were changed to high accuracy multi-turn types for better accuracy.

Here are the Marchand XM1 specs :

Frequency response: DC to 100 KHz, +/- 0.2 dB
Crossover frequency choice: 20 Hz - 20 KHz
Insertion gain: 0dB ( 1X )
Filterslope: 12/24 dB/Octave
Harmonic distortion at 1KHz: less than 0.01%
Signal to Noise ratio: better than 110dB
Input impedance: 25K
Output load capability: 2K min
Output impedance: 100 Ohm typ
Maximum input voltage: 25 V peak-peak (8.8 V RMS)
Power supply requirement: dual regulated +15V and -15V @15 mA, typ

Power is supplied by the Marchand PS10 +/- power supply.
Time to build the box - I'm using 3mm MDF, coated with mdf-primer & then sprayed on matt black enamel.
The VU kit meter took about 3 hours to construct .
Switches on the front allow me to monitor either the main speaker power levels, or the subwoofer power levels for both the left or right channel.
Sensitivity has been set so that the red LEDs come on once 29 volts (100 watts into 8ohms) reaches the speaker terminals.
The power switch is located on the right hand side to avoid accidental (& potentially harmful to speaker) use.
The power levels for each LED are as follows :
This has turned out to be a really useful meter which I highly recommend. I find that most of the time I listen at about 1-2 watts (peaks) which equates to about 80-85dB SPL at the listening position. Even at painfully loud levels I am yet to reach orange levels (60 watts) to either the main speakers or subwoofer!