It’s always interesting for me to learn about some of the reasons why players adopt tuning in fifths on the double bass. The following comment just came in on one of my classic (i.e. old) blog posts about reaching the low notes:
I also use the fifths tuning. Joel Quarrington’s website persuaded me to try it, and my experience has borne out everything he says (from the other end of the food chain – I’m an amateur orchestral player.)
Not only do you get the low notes without the problems of either 5 strings or extensions, but the bass resonates and sounds far better.