TubeNet

They are playing the modern version of the french Saxhorn Basse in Bb, these are compensated like the euphonium but less wide. So they are much more versatile in tone and colour. It is interesting that the ensemble switched from the Courtois (with the thumb-operated trombone-style tuning slide) to the Willson Willsax in the newer recordings. This is a nice playing instrument, but very uncomfortable to hold, having the fourth valve not longer between main bow and valve tubing.
But back to the french C tuba: I am looking forward for new built instruments, but there will be a lot of disappointed buyers afterwards. It may look like any brass instrument, but playing euphonium means not being able to PLAY a french C. I have a very nice picture of Brian Bowman playing my Courtois C, looking very embarassed about his sound. And he had his own mouthpiece... You have to spend the same time on it for the right sound and clarity as you would need for taming the ophicleide. Because it is just a better Ophicleide...
It would be interesting to have a C/Bb version like Couesnon offered with their "swan-neck", pictured on title of Clifford Bevan's Tuba Family, first edition. The character of the instruments changes totally with the extension.
Keep on blowing!
Helmut

Very interesting, thank you. You can certainly hear the four bassoons in the lower octave.

What I take from that video is that the brass have that bright and 'immediate' sound - generally associated with the French - yet the string parts sound more prominent because they're not got buried over by powerful 'modern' brass. Is that a fair assessment?

Yes, that's right. With one of those "pea-shooter"-trombones or the narrow-bore piston french horns (the french french horn!) you would be unheard in a modern orchestra's brass group. I tried it once as second trombone in a Schumann symphonie, the first didn't use the alto but a Bach 42. He had to play my solo...