Not sure this would get this sound right because any Trueplay on the Era’s would not factor in the subwoofer. Technically I see no objections grouping the Connect Amp with the Era’s. You could try this and return the Era’s is you would not be satisfied.

looks like i can only group and not flat out add the amp to the era. is that right? i don’t want to group everytime i play something so that would be a showstopper. 

Correct, the CONNECT:AMP can not be in the same Sonos ‘room’ as the Era speakers, it must be a grouped ‘room’. Sonos has never provided a solution to add a third party speaker to a room that isn’t driven by an Amp or CONNECT:AMP.