I think the definition has been blurred by market changes- in the early '70s, an integrated amp was power amp plus mostly full featured preamp with tape loop, sometimes tone controls, and other features characteristic of preamps at the time-- possibly even a phono stage. With the rise in popularity of digital sources and a range of tube amps (I'm more familiar with those than SS) containing volume controls and often more than one input with selector, such an amp can make a separate preamp stage unnecessary. I'm not sure there is a single definition of "integrated amp" for this reason--I used to think of them as "receivers" without the tuner section. My take, fwiw.
