I've used cold light heads for almost 25 years. Before VC paper improved I used a De Vere with graded papers and of course I had no compensating timer. My experience then was that unless I kept the enlarger at a consistent temperature the final print did not match the test strip. I overcame that problem by leaving the enlarger on as I developed the test strip and then switched it off to load the paper to make the print and it did work although it was some what crude.

I now have two Zone VI VC enlargers with RH Designs f Stop Timers and cannot fault either. The compensating timer takes care of the fluctuations in the illumination and has a number of other excellent additions like, 2 channels for split grade printing, the ability to change the f stop setting fron 1/2 stop through 1/3, 1/4, 1/6, 1/12 and 1/24th stop increments as well as a very good dry down feature.

I have also seen the new version of the Zone VI enlarger when I visited Richard Newman a friend who works for Calumet in California. The light source is LED and it is very good and I think it is now available.