Can anyone recommend any dixie/traditional/swing jazz bands who are youthfull (not necessarily young) and prefer jamming and improvisation as opposed to well rehearsed sets ?

I was at one of the farewell party of Herb Miller, a legendary Swiss band. Although lacking in spontanaeity, they were highly entertaining and versatile. Their repertoir included not only New Orleans classics, big band numbers. Because these are their last shows (tonight as well if you're interested), they have added some beyond jazz fun stuff as well (e.g. Herb Alpert, rock & roll, Ländlermusik). Previous members of the band were also given the spotlight. I was hoping for an almighty jam session at the end with all the musicians present and past, but it didn't happen. I enjoyed the evening but I'm now yearning for some originality and 'real' jazz.

The concerts that have most impressed me in recent times were

The Blue Birds of Paradise at the Salzhaus in Brugg.

The New Orleans Hot Shots at the Laufenburg Jazz festival this year.

Breiti & Wark at the Baarer Altstadt festival.