I have a Durst CE1000 with a CLS450 head and power supply with a 250 watt bulb installed. With the 6x6 mixer box installed, my exposure for Supra Endura on an 11x14 print is 6-8 seconds at f11 with a Componon 50 f4 lens which dates from 1963. When I use a 90 watt bulb, I open up the aperture about one and a half stops.
It almost sounds like the filters are all fully in the light path all the time. Does the light change colour when you adjust the filters one at a time?
With all three at max, you have a pretty dark image, whan all are set to zero it should be very bright.