I believe that The Mellstock Band still use a serpent.

The do indeed. From hearing it at close quarters, my impression is that it's quite resonant in the lower end, but seems to get a little weaker in projection in the higher notes. It was superseded by the ophicleide and similar instruments. The Old Swan Band use a bass sax in their lineup - and that's a really powerful instrument! I played in a jug/jazz/juke band in London, sitting next to a bass sax player. He told me that he had to anticipate the beat and blow a fraction in advance of when the sound was needed - such was the length of the tubing!