Hi Leon
I went downstairs and took another look at our Durst.

In the CLS301 head, there is a filter drawer, its just under the colour dials. Is this drawer empty?

We have 3 mixing boxes, Medium format, 4x5 and 5x7. The 4x5 is quite large, 4 5/8" x 5 5/8" and 6 1/2" deep.
The sides of the mixing boxes are made from very shiny aluminum with 1/8" thick white plexiglass in the bottom.
The plexiglass is scored with very fine (faint) circles radiating from the centre of the box.

We did have to replace the light sockets when we first got the enlarger as they were black.

What type of film holder are you using, the glass (one size fits all) holder (which uses blades to mask to the edges of the film, therefore putting maximum light through the film itself) or film holders sized to the shape of the film?

Using a Sekonic Light meter on ambient light setting, ISO 100, with the white globe of the light meter 22" directly below the lens, using a wide open lens, aperture 5.6, the light meter exposure reading shows 8 seconds. This type of reading will tell you the strength of the light.