Unless your amp has an integrated DAC (like the Peachtree units, for example), the amp itself is irrelevant to your question. The Sonos Connect/Zone Player 90 has two methods of output: analog and digital. The digital outputs do no conversion. So if you output digital data from the Sonos it must be converted to analog before your amp can accept it.
If you use the analog outputs of the Sonos unit, the data has already been converted to analog by an integrated DAC chip in the Sonos. So your question is whether an external DAC will improve on the audio quality, compared to the embedded DAC module in the Sonos.
My opinion is yes, an external DAC will almost certainly improve the sound. I would advise you to test an external DAC and ascertain if you can hear a difference. But since the Sonos will only output at 44.1 kHz, there is no need to spend too much on a DAC. Also, you won't need a USB DAC - SPDIF or optical will do unless you want to complicate life and use an optical or SPDIF bridge.