The U.K.'s Flotilla is an excellent saxophone quartet, with a good tone and interesting repertoire - although I must be open and say that the leader is a friend of mine. Their CD got an excellent review in Saxophone Journal, here is the link:
http://www.flotilla.org.uk/Flotilla/Home.html
As you mention players with a pure tone Kyle Horch (an American player who lives in London and leads Flotilla) has such a tone and you may find his playing will help your students. He is also Professor at the Royal College of Music. if you want a preview of his playing, here is a live recording (playing one of my works):
Islas - video
http://www.flotilla.org.uk/Flotilla/Home.html
As you mention players with a pure tone Kyle Horch (an American player who lives in London and leads Flotilla) has such a tone and you may find his playing will help your students. He is also Professor at the Royal College of Music. if you want a preview of his playing, here is a live recording (playing one of my works):
Islas - video