Your post is not very clear. There is no such thing as an ‘ARC playbar’ or ‘amp playbar’ , and a few other things that don’t make sense, so I can only guess at what question you’re trying to ask.
With a Sonos Arc, you can setup a Sonos amp to power two passive speakers (normal speakers connected by speaker wire) for surround sound duties. When used for rear surround, the Sonos amp cannot also connect to a 3rd party sub. You can only use a Sonos sub. You can’t use a 3rd party amp (non-Sonos) for rear surround speakers either, and thus they idea of using a 3rd party amp to control volume would not apply. Volume control for the your home theatre setup is essentially done by Sonos...you would be able to control volume with the Sonos app, voice control, or through your TV remote (or other remote depending on specific setup).
Apologies for any confusion created on my part - new to Sonos here. I should have just said an ARC soundbar rather than the (Sonos product term ‘playbar’). I was simply trying to distinguish between ARC (product) and ‘ARC’ HDMI protocols that, I understand, are available on both the AMP and ARC and get talked about in relation to using the AMP. That was a bit confusing to me.
Anyway, thanks for clarifying that the AMP will not support both the passive speakers and the subwoofer to the ARC. Seems to me that this is a ‘big miss’ given that the AMP has the sub output built in. If the ARC can provide wireless output of a sub channel to the SUB why can't it do the same for AMP? I can't see that the AMP would need any additional hardware in the box to do this and it’s clearly intended that the AMP can normally support 2 amplified outputs and a sub line-out. Maybe I’m missing something here but it’s hard to find that in the documentation.
Unfortunately, I think that this effectively kills the AMP option for me. It’s simply going to be quite a bit cheaper to grab a couple of ONE SLs sometime and maybe a SUB (and sell my rear Linns and Sub). I won't worry about the AMP being on backorder and I’m glad I did ask before buying :-)
Thanks again for your prompt response.