You can use your own speakers and a Sonos Sub when the Amp is used in surround configuration with an Arc. You cannot use a non-Sonos sub in this configuration. I would not consider grouping an Amp with an Arc just to use a non-Sonos subwoofer. Certainly not an intended configuration.
When Sonos devices are used for surround speakers they connect in a fundamentally different way from when used as principal speakers. For technical reasons lines -in and the wired subwoofer output cannot function in this situation.
IMO there is little or no chance of this changing.
So in order to use the non-sonos in-wall speakers for home theater, i would need the Amp. But to use a sub, i would need to buy the Sonos brand sub, correct? Again, all for a 5.1 home theater surround experience (even though the front three is the arc)?
thanks