that is indeed a difficult question - if you want to come up with something truly remarkable and original in 1971, you've probably been working on it for a good portion of the sixties, too... that holds to a lesser degree for less original music... i am far from a brötzmann expert but his 60s output includes
For Adolphe Sax (1967)
Machine Gun (1968)
Nipples (1969)
More Nipples (1969)
Fuck de Boere: Dedicated to Johnny Dyani (1970)
would agree though that brötzmann's music had a decided "the sixties are over" appeal already on Machine Gun (roughly, taking the music of Albert Ayler and replacing the spiritual side with beer, tits and automatic weapons)
here's another supporter of Soft Machine btw