Jeff, on your comment about "3 string basses tuned GDA, etc." do you have the G-string on top or bottom in 5ths? I think in England at least it was 3 of the 4 strings we have now with the A on the bottom all in fourths. Over 99% of the Orchestra players in the world (maybe less) use fourths with or without an Ext. or a 5er to Low B or C. If this 5ths thing was a better option, more than just one or two players would have switched by now.
Next factor is the Bass. Many of the Basses we play today with 4 high tensioned steel strings in fourths were originally made for 3 lower tensioned Gut strings in 4ths or some other combination.
Many Basses today have sunken Tops from the extra pressure of 4 steel strings. Joel has a fine old Bass but I'm am not sure if it is a G.P. Maggini, a Santo Maggini (not related) of some other nice old Italian or even English made Italian copy. Regardless of who made his Bass, it is a 'smokin' instrument. I don't know if all the Basses out there could be successfully converted to 5ths and hold up structurally. Playability is the other factor. The Bass is hard enough to play as it is. Many a concert I shook my hands afterwards from fatigue.
I did recently and a long time ago as well tune my Bass in 5ths to give it a try. I was LOST!!
On d-tuning, I have also tuned down to Eb, D, Db and C on occasion and it was no picnic. I had to put fingerings in the music to remind myself where to play the notes up to the Ab before crossing over, etc. Also, I had to tune down and up during the music and count the turns on my tuner that I rehearsed. I look like the guy in the section fumbling with his Bass when in actuality, I'm the guy that is actually playing the written part correctly.
Currently I am working out this summer on my 5er to get ready to play Beethoven's 6th the Storm movement. It is only for a few passages I need the 5er being that the Extension is a lot of jumping and intonation is just about impossible. It is more likely that I will get injured trying than not. The 5-string is most likely what the piece was written for and not a 4-string tuned in 5ths, I think..