I wil check out Coles Notes and dig around over the weekend.
Here is a great example of Canadian-isms. "Coles Notes" are the Canadian version of things like "CliffsNotes". They are a type of study guide that cover the important bits of subjects such as literature, usually in a very condensed fashion. Hence me saying "the Coles Notes version is that just about anything is possible." While I would be extremely surprised if Coles Notes had anything on the double bass or tunings for it, a fantastic resource mentioned over and over again here is
Paul Brun's book
A New History of the Double Bass. It is much cheaper than a lot of other reference books mentioned here, has an entire section on different tunings among other very interesting topics, and the Contrabass Shoppe in Walton on Thames in Surrey, England has it for sale for £65. You might be able to find it in a library somewhere as well.
I would say everything below a low B is rumble. A C# string is useless and over the top in my opinion.
In the context of a double bass, once you get down to a low "A" (an octave below the A string on a 4ths tuned bass) you are really pushing it. I have seen it done a few times successfully, but very few bassists and fewer instruments are capable of producing it in a pleasing fashion. Likewise, that string becomes almost useless for anything other than long pedals, as you can forget about articulation down there. Although modern string technology has helped a lot, the dimensions of the instrument and string length and other factors are huge barriers at that point, as some instruments even struggle to produce a good low C.
If you search for "extended range" over on the electric side, you will find that there are heavily entrenched sides to the debate. The spectrum of human hearing doesn't really go much lower than the A I mentioned earlier, with "optimal conditions" being down to a C an octave below the 5ths/extension C. Your brain ends up filling in the blanks based on the various overtones produced and thinks it is hearing notes in that range, so there is an argument for those extreme tunings. Then you get into talking about having strings, instruments, pickups, effects pedals, amplifiers, cabinets, headphones, monitors etc. and whether or not they can faithfully reproduce those frequencies, and it all goes downhill from there.
I do not see myself going below a low A on double or electric bass any time soon. I will use the low C of 5ths tuning when it is called for, but I have matured past the "play absolutely everything possible down the octave" stage that tends to happen when you first have access to those notes. While low F's do pop up once in a while, very little of the repertoire actually uses that low C string in 5ths. I have a 5 string electric that goes down to a low B, but it really doesn't get a lot of air time either. Used sparingly in the right context those low notes can be extremely effective and turn some heads, but if you start throwing them around all over the place they aren't as effective.