Your home theatre system is one Sonos room, not a group. The Subs and surrounds are bonded with the Arc to play together as one room.

You cannot add any further speakers/devices to your existing home theatre setup. You have reached the max. for that room.

Ok, would be nice to do that. A new feature in the Future maybe?

How would you use the additional amp in the home theatre room?  Additional front speakers?  Side speakers?  Atmos speakers in the ceiling?  I don’t see Sonos allowing for any of these configurations as the Arc is suppossed to cover these needs with the upfiring and side firing speakers.    As well, you’re intrudcing additional wireless communicaiton required for the room, which reduces the possiblity that everything will work smoothly in every room situation.

Well, before you could not add more than one sub in a room, now it’s possible to have 2.

So limitations have changed.

The amp could be used power another SUB for even more bass.

Strictly speaking, the Amp doesn’t power a Sub, unless you use the L/R channels as input to a passive sub, which is not the original design.  Instead, the sub out is a line level output for a powered Sub.  In addition, when used as surrounds in a Home Theater configuration, the Amp’s subwoofer output is disabled.