1. Can I use a Logitech Harmony remote to control the Sonos Amp to control volume and change audio source?

Yes, the Harmony remote, along with the proper settings in the Sonos app and Harmony app, will allow you to control the volume and change the audio source.

2. Can I connect an SVS PB-1000 sub to the Sonos Amp?

Yes. https://support.sonos.com/s/article/4953?language=en_US

3. Can I add Sonos wireless rear speakers to the Sonos Amp to make it 5.1 surround sound?

You can add wireless Sonos speakers as rear surrounds but the maximum setup you can achieve with an Amp is 4.1.

https://support.sonos.com/s/article/4807?language=en_US

4. Is it possible to connect a Sonos ARC or Sonos Bar to the Amp to make a 5.1 setup?

No. The Arc, Playbar, and Beam cannot be added to an Amp setup, BUT you can use an Amp to add passive rear surround speakers to a Sonos soundbar for a 5.1 setup.

FYI… the maximum setup you can have with a Sonos Amp is a 4.1 setup. The best audio the Amp supports is Dolby Digital 5.1 (with a phantom center channel).

5. When I use Spotify or Tidal on the Sonos Amp, does it play at the full audio quality bitrate or does it scale it down because like it would do if using Bluetooth to stream the music?

Sonos supports lossy compressed audio up to 320kbps and lossless audio up to 24-bit at 48kHz (through Qobuz). So you will get the highest quality audio Spotify Premium can offer (320kbps). With TIDAL HiFi, you will get 16-bit/44.1kHz. With TIDAL Premium, you will get 320kbps.

Sonos does not support MQA audio from TIDAL. The Amp also doesn’t support Dolby Atmos audio from TIDAL.

6. In a perfect world I would like to have an actual centre speaker for movies and tv shows. Is there any way to use a Sonos Amp with my KEF L/R speakers and also have an actual centre speaker? Can I add a beam or something as a centre speaker?

A center channel speaker cannot be added to the Amp. A Beam or any other Sonos speaker cannot be added as a center channel speaker either.