It is very easy to determine where the problem is.
Exchange the left and right speaker cables at the Sonos ConnectAmp (or the amp that the Connect is using).
If the amp is bad then the other speaker will now play low.
If the wire or speaker are bad then the same speaker will play low.
Next step is to remove the speaker and connect it directly to the amp.
If it plays low then the speaker is bad (maybe the woofer is bad).
If the wire is bad then you will have to run a new cable. A partial short in the speaker cable could cause this. Your installer should be able to check the cable with a meter to determine this.
Sonos gear is pretty reliable but if it is the problem they will replace it for you under warranty.