What speakers are you using with the Amp? What specifically are you listening to?

I’m running B&W’s WM-6 outdoor speakers and music I’m mostly listening to rock country. 

You must be used to amps that deliver a lot of their power early on in the volume control movement; a trick to appear powerful. Sonos amp does nothing of the sort, and half way up is fine, especially outdoors where sound disperses rapidly.

If you get the sound levels you want at 70-80% on the slider, nothing to worry as long as the amp is in a ventilated space.

It is worth checking also on the Sonos app that the amp is set to 100% volume levels.

It could just be that you are using speakers outdoors. Since outdoor speakers are often used in a pretty open space, the volume has to be turned up pretty loud. My Amp powers my outdoor speaker and I usually keep the volume at about 50%.

Have you checked your speaker connections? Have you adjusted the EQ settings and turned Loudness on? Also be sure that Volume Limit is turned off.