MMB: This is not simply a 4-string cello with a fifth string; it is an absolutely new design of instrument. I didn't imagine the adjustment would be quite so dramatic. 
The five strings give the cello a wider tonal range and variety of modulation. The E-string gives the instrument a very warm sound, rich in harmonics, which carries well in a concert hall.

As far as I'm concerned, the fifth string simply belongs to the cello. This is my personal opinion. I wouldn't exchange it for any Strad unless it had a fifth string!

CELLO MASTER COURSE - J. S. BACH 6 CELLO SUITES / 5- & 4-string
Sachrang · 18.-23. august 2015

Pablo Casals:  
"They are the quintessence of Bach’s work as a composer -
and Bach is the quintessence of all music."