What Reid said is true for tube amps. People generally buy tube amps because they want the tainted sound of a tube amp, and don't trust a digital EQ to do the same job for free. It's a bunch of wank and is entirely personal preference. It is also 150% not accurate audio reproduction. People buy it because they like the tainted sound, not because it's correct. Tubes also have a bunch of other problems (like, you know, burning out).
A serious digital amp and DAC combo can on paper do a better job of audio reproduction than most receivers. Does it matter? Typically, no. Especially if you are not driving them to ridiculous volume levels. Distortion only becomes apparent as you drive the receiver towards its limits. That's why ridiculous-scale home theaters use receivers paired with a separate amp, because they just straight need more power. It's not that receivers these days can't produce good audio, they absolutely can. But they won't drive 12 speakers at 100W cleanly.
All that said, 2 speaker stereo on a receiver like that mid-range Denon is ABSOLUTELY within its wheelhouse of capabilities and you would gain nothing by getting a different amp.
As for the rest of the BS they dropped on you, bunch of wank, just like Jon`C said. The Chromecast Audio over optical cable (and the video one, via HDMI) is compatible with bit perfect reproduction of audio. That means it's giving the receiver exactly what you're playing, with no analog conversion, no compression, no adaptation at all. You can't get more perfect than that.
The receiver is not going to do jack **** to the video quality, unless you turn scaling on (which is a simple on/off in settings). It will be perfect. Even $25,000 home theater insane setups still use receivers for video, and for audio DACs. Because they're still fantastic for that purpose.
Also, Divx software players are piles of ****, and I honestly would have walked out the store if they suggested that crap to me. What a joke. That's like suggesting Internet Explorer to someone as a good browser.