Sad to hear.  The Doobies - along with bands like the Eagles, Steely Dan, Earth, Wind and Fire (and a few others) made virtuoso music that was radio-friendly and accessible.  One of my earliest rock and roll surprises came when I unwrapped my copy of Toulouse Street ... that gatefold shot with those beautiful, topless women!  But I digress.  Rock on "Little John".

I saw the Doobie Brothers several times in the 1970's. Remember Little John on one of the drum sets well. 

That band had good commercial success and several huge selling albums and a few top singles. 

But they would change so much as the years went by. Almost as much as Fleetwood Mac. 

Last time I saw them with Michael McDonald on keys and doing much of the singing and Jeff Baxter on many of the guitar solos, it was a very different sound and band than the early and mid 1970's. 

@robertdee I remember them being referred to as "Doobie Dan" when Michael McDonald came aboard- a nod to McDonald's participation and sound with Steely Dan.  Yeah, they were almost like two entirely separate and different sounding bands -both with amazing songs and chops!  It's all good!

@rusty Yes Rusty. Fleetwood Mac is another band which comes to mind that transitioned into such a different band and sound. 

Clive Davis insisted the Allman Brothers do that to a large degree in 1980 and they tried to be more of a pop/hit singles band especially on Brothers of the Road but it just didn't work for them like it did for the Doobie Brothers. I'm not sure why but for the ABB it was kinda like the Hell's Angels moving over to Honda motorcycles and dressing in red and orange pants and clothes like the guys in Van Halen wore a lot. It just wouldn't work. 

Some of their story.