I have an Echo Plus in the Den. It controls lights (Philips Hue) that are in the den. I do not have a Philips hub, the Echo Plus uses its own built-in hub.
I want to have an Echo Dot in the bedroom. Will I be able to control the lights in the Den from the Echo Dot (via the Echo Plus' Hub) ?
You should be able to control any smart device you have connected via an Echo plus with the other Echoes. All the Hub allows you to do is control devices that require a hub cutting out the middleman. As long as all of your Echoes are connected to the same Amazon account and a Wifi network they should all control your smart devices. If you find issues with it contact Amazon’s customer service they call you write back.