port vs era 100

I don't use the inputs but from reading stuff here i think you might not need to mute the Era, just don't select the line-in as the source. I think you could even play somethig else on it while using the line-in there as a source for a different Room.

Sonos doesn't publish much in the way of specifications so numbers aren't available but the impression I have is that any Sonos line-in is going to be of similar quality.

I set notifications on ebay for an S2 Port and found one in excellent condition around or under $200. 
There’s also an S2 Connect often on there for a lot less. If you are in a hurry there’s never a deal but if you’re patient you can find a deal. 

There’s a thread on here where I was struggling with setting up my living room because in Home Theater Mode because you cannot use the line-in on an Era100 it is disabled. 
So the only options sadly were Port or use a silent Era 100.  I reluctantly bought a Port. 

Bummer because I bought into Sonos based on using it with the Pro-ject TT they sell. 

As ​@Stanley_4 says, you can still use the Eras as speakers, even when the Era line-in is used as the input in a different room. You have the kit already, so try it!
 

I believe that a stereo-pair of Eras with line-in adaptors connected to each speaker means that both sources are available simultaneously to the whole-house system.