You may be able to use one that can be wired direct from the main L+R speaker outputs of the Sonos Amp. Some subs allow this. (REL for example)

Actually, the Sonos Amp has a dedicated subwoofer line-out.  You would connect that to the sub amplifier’s line-in. 

https://support.sonos.com/en-us/article/use-a-third-party-subwoofer-with-amp-or-connect-amp

Hello 

i recently purchased Definitive Technology IWSub 10/10 in wall sub woofer and connected to my sonos amp but the outcome is very poor

i was told i need to add Third party subwoofer mono amp to my sonos amp but i can’t find any details or what mono amp to get in order to get the maximum power to the subwoofer 

please if possible guide me through this on how to connect the 3rd party mono subwoofer amp to my current sonos amp.

also if possible please recommend me of what mono subwoofer amp to purchase in my case to connect to my sonos amp to power my Definitive Technology IWSub 10/10

much appreciated 

Moderator edit: moved from duplicate thread

Definitive Technology would be better able to suggest a subwoofer amp.  Once you have the subwoofer amp, you would wire it with speaker cable to the sub, and an RCA to RCA cable from the Sonos Amp sub out to the line-in on the subwoofer amp. 

A key point is that the Sonos Amp subwoofer output is a Line-Level (1-2 Volts) output that can barely drive headphones.

A power amp or amplified subwoofer is essential.

From folks that know that sub: 

https://www.crutchfield.com/S-GGn5rheqsJn/p_735IW1010/Definitive-Technology-IWSub-10-10.html

designed for use with Definitive Technology SubAmp 600 Subwoofer Amplifier (sold separately, required for operation)

Sadly, not currently selling the 600 there but have a good page for it anyway.

https://www.crutchfield.com/p_735AMP600/Definitive-Technology-SubAmp-600.html?s=0