You may be limited to the sources you can play back
FWIW, if I'm not mistaken, the xx13 models removed some of the ability to send along digital sources to zone 2 & 3. Read the following thread and see if it applies to your needs. I also do not believe anyone successfully answered whether or not Airplay functionality via PCM would be an 'end run' around this limitation. Might be worth popping over to AVS and seeing if anyone has any suggestions.
Also see p.104 of the Denon manual online for footnotes about what types of signals it can pass to zone 2/3 via pre-out. IOW, it might not be as simple as adding a four channel amp for your situation/hardware. Crutchfield mentions this limitation here too under 'multi room audio video'-
If you can find a compatible receiver though, I like the following Dayton multi-channel amp @ Parts Express. It's about as economical as you are going to get.