You can connect a bluetooth adapter to your amp and pair it to the dot. However, when you connect the dot to a bluetooth adapter or speaker, all audio will be directed to the bluetooth device. The wired connection also shuts off the internal speaker.
I have a similar setup with the harmony elite. I am connecting a dot, using a wired connection, to my old receiver, which powers speakers in 3 rooms. I will have a new echo in the adjacent kitchen (arriving Wednesday), so that will do all the normal echo functions. The dot will just be for the harmony hub functions, and I know I will not get any audio responses from it if my receiver is off, but I can turn it on with voice commands. This will allow me to play music through throughout the first floor. "Alexa, turn on echo" will have the harmony hub power on my receiver and set the input for the dot. "Alexa, play XYZ artist", or, if in kitchen, "Alexa, play XYZ artist in Living Room"* will start music. If my TV was also connected to the receiver, when I would tell Alexa to turn on the TV, it would switch off the dot input and switch on the TV input. In my case, I have a separate soundbar for my TV which will then be switched on, and I could keep the dot audio to my other speakers on if I so desire.
Should work in theory, I'll soon see how this works in real life.
*note: the playing of music to another echo does not currently support Spotify or SiriusXM, but is supposed to in the future.