Power Amp with digital input?

I'm assuming that $500 is for the dac/amp, and not speakers. If that's the case, have a look at this. I think its exactly what you're looking for.  

http://www.psaudio.com/products/sprout-elac-system-2/

If your intention is to just have speakers and the Sonos, have you considered powered speakers. Audioengine,  Vanatoo, Paradigm to name a few. 

I'm not familiar with the SONOS CONNECT but I have had my Bluesound Node 2 directly connected to my (analog) amplifier and it worked beautifully. Node 2 makes it possible since it allows you to control the volume, tone, and even has a subwoofer out. So it basically acts as a preamp. Of course this set works if you have a single source. Another important consideration is the relative input/output sensitivities of the source and the amplifier to give a decent listening volume.

Lyngdorf, NAD and TacT, amps have digital input, but you might find the $500 max a stumbling point.

" An integrated amp is overkill, with more inputs and controls than I want."

You don't have much of a choice. If you put a dac in a poweramp, the only way to get a preamp is to have it built into the amp. Since most people have more than one source, an amp like you want would mean losing a lot of sales. Its not practical. Just get an integrated with a built in dac. Otherwise, 99% of your available choices disappear.