I guess it is almost like a combination of all the smells I listed (wet dog, hot spots, rotten) but not really more like one or the other.
I have smelled it before on other older dogs when I worked at the vets office & most were cured by shampoos & antibiotics, some passed before we could figure it out & unfortanatly some owners did not want to try to treat the smell.
At first I just thought "oh well an old dog smell" but this is getting bad.