Just use the one on the Sonos Amp...and if you don't trust yourself or others, set max volume level on the amp lower than 100 percent via the Sonos control app.

In general, an amplifier that *can* put out more power than the speakers will accept (max.) does not mean that it *will* put out so much power.  So you can use your system without worrying, because you are unlikely to keep increasing the volume after you hear the speakers distorting.  (It isn’t subtle, you’ll know!) 

However, if the system is at a party venue to be run as a professional sound system -- or on a rental property, where people are likely to just “turn it up to the max” without regard to whether or not the speakers are making crazy noises -- or similarly, where the volume control is nowhere near the sound of the speakers (which never need be the case with Sonos!), then you can limit the max volume as @Kumar suggests.

eEdit: At 95 dB/1W/1m, normally one would say it will get way too loud for you to listen to, long before you near the speaker’s max at 85W.  But sound outdoors is never “as loud as you expect” like indoor speakers enclosed in a room.  So you might want to set a maximum in the Sonos app, even if you do trust yourself and others: Go outside, near one of the two speakers for a zone, and listen *nearby* as you increase the volume (while playing modern dance music or whatever with low dynamic range just loudloudloud all the way thru).  Back off just as soon as you hear anything suspicious, and that should be your max, no matter how loud it (doesn’t) sound down on the patio or wherever.  Of course if you’re just using it for background new-age garden tinkling then you don’t need to bother with this at all.]

This is one of the most difficult questions to answer because there is no firm answer.

In the case of a lightbulb, a 100W bulb will always draw 100W at its rated voltage. One can continue to add bulbs to a circuit until the maximum current is reached. Beyond that point a fuse should blow.

I know that the consumer would like to feel that an amplifier does not have enough power to damage the speakers. Speakers are similar to an automobile where current road conditions determine the safe operating speed, not the posted limit or the top number on the speedometer. Another difficulty is that there is no agreed upon standard used to assign a maximum power handling capability for a speaker. The speaker marketing department has the most sway when assigning a maximum power limit to a speaker and cheap, inferior speakers often tout higher numbers than quality products in an effort to sway the purchaser into assuming that the cheap speaker is the better product.

It doesn’t seem fair, but small amplifiers are more likely to damage a speaker than large amplifiers. If someone is habitually burning up tweeters, a more powerful amplifier is the best solution. If you give me a job “please blow up this speaker”, I’ll pick an amplifier in the 50W range and drive it at 80W or more. The amplifier will be very distorted at this point and rich in high frequency energy that will blow up the tweeter. An 80W amplifier running at the same level would not usually damage the tweeter. My last choice of amplifier would be a SONOS AMP because they are well protected and will not enter this high distortion mode.

Your outside “room” is quite large and with this equipment you will not be able to achieve rock concert levels, but if you follow tracker’s suggestion, you’ll be fine.

I’ll also mention another awkward detail: we are conditioned to equate ‘loud’ with distorted. We keep raising the Volume until we perceive distortion and accept this as ‘loud’. As I stated above, it is the distortion products that usually damage the speaker. In my college apartment I could run at levels where verbal communication was very difficult, but the sound was clean. Regardless, I would get requests yelled in my ear to “turn it up” because it wasn’t ‘loud’ yet. The same crew in another apartment, using an awful little compact unit that was pushing out horribly distorted sound, accepted that unit as playing ‘loud’. Conversation was easy. In this context AMP will never seem ‘loud’.

The problem with outdoor speakers is the lack of surrounding walls, and the physics of how sound dissipates with distance is then allowed full scope. Hence minimising the speaker to normal listening positions outdoors is very important, even if it means longer/thicker speaker cable runs up front.

All that done, if you find that the max sound levels you need in the listening areas are met with the Amp volume slider not beyond about 75%, you should be ok. Then it is just a matter of setting the max volume on the amp at those levels and playing within the recalibrated limits of the volume control.