The bass player I am referring to also tunes his bass in cello tuning. (I also know another guy that plays upright in 5ths but bass guitar in 4ths. Now that would be a challenge!)
I am not casting aspersions on anyone. I just am curious as to advantages of tuning a bass this way.
Extended range in both lower and higher register without having to play a 5 string, that's why I've put that Wax'In video cos' it focuses on Philippe Bussonnet fretboard many times, the bass is mixed very upfront. You can clearly hear really low C to D notes, and also that "false 3rd interval" fingering when he plays an augmented 4th.
Tuning of regular EADG in Hz
E 41.2
A 55
D 73.42
G 98
J. Top and P. Bussonnet tuning cello type but an octave lower than cello:
C 32.7
G 49
D 73.42 , same as the regular EADG for instance, and that gives a feeling of having 2 distinct ranges in the same instrument as P. Bussonnet already mentioned in an interview, a really low one and a more "traditional bass" one, that's why he switches a lot.
A 110
Also, it helps for prog and avant garde music genres as the chords and the changes are really tricky sometimes, they are not "mainstream", they found out it was easier to use that kind of tuning with those kind of musics.
Now yes it requires to play like a double bass player, or a cellist for instance, you have to switch a lot when soloing (the KMX B12 solo video), so it can be challenging, but also rewarding cos' you have to work in positions for only 1 or 2 strings, believe me: you know way better your bass after that than playing pop in regular 4ths on the first 5 frets !