Jim said it all.....the sound changed because they grew up!
Can hardly play that wild thin whining echoey stuff like Man of Mystery when yer playing pantomime to the varying age groups, I 'spose...
They were changing and maturing into 'all round entertainers', with panto, song and dance, daft costumes, mild comedy routines etc and appearing on middle of the road mainstream TV shows with Cliff etc, and adult nightclub venues like the Talk of the Town etc..
That era after about 1962/3 was basically their new mature career direction- and I'm not knocking that, they were still successful after all.
Just not really to my taste, that's all...sorry....(and from what I've seen of the latest tour footage, they have followed that direction to its ultimate conclusion- a smooth, polished ultra professional top class entertainment show.)