1. would I combine 2 Russound units to achieve my 8 zones, or should I be looking at one 8-zone unit to achieve my 8 zones? Or would I combine 2 zones to work with the suggested Russound (as long as I can still install 8 wall plates for the sake of switching on-off / volume control all 8 zones individually)?
Yes, you would get two of those units. They offer a connection capability which allows the four sources to be shared by a total of two Russound units which means 4 sources to up to 8 rooms. You wouldn't double up on rooms, but would dedicate one speaker pair per room/zone. In an area with more than 2 speakers (outside?) then you will need to use an external amplifer for that one specific area and it must be on output four of the Russound.
2. Is a Russound unit set up in such way that each zone could select it's own sound source (which would be great and more than I was hoping for!), or would this entail a further 7 amplifiers when reading through the Russound description? Could, say, bedroom 1 listen to Sonos whilst bedroom 2 tunes into the radio? What happens when bedroom 3 starts tuning the radio too on their wall pad when there is only one tuner?
Yes, each zone would have a keypad and would independently be able to turn on/off a source of their choosing and adjust their volume locally.
Each source is a single item, so if you have multiple rooms using the same source then that's what that source is. If you want more sources than 4, you will need a different product, and you should expect to pay more. I use a 16x8 source product for my home. Lots of sources = better IMO, but it has a hefty price.
3. I take it that some Sonos system would just plug into the Russound unit as another source, but ultimately controlled via some app / one Wifi device controlling the actual content on that “channel”?
Any Sonos I believe can plug in as a source, and control of the Sonos itself can be run through a iOS/Android device. Likewise, if you have Apple stuff, you can throw an AppleTV w/Airplay onto it if you want. Lots of options.
4. I will check on Russound whether any mini-jack or maybe even USB can connect (I mentioned pad, phone or even as simple as a Walkman / disc player)
USB is not a audio source, it is a data source. Some products now support reading certain data streams from USB devices, but it is not a 'standard' under any circumstances at all. If you want a standard audio connection, then that would be a 1/8" mini jack or two RCA plugs.
5. I take it that the remote would be the only central control when opting for Russound which would access the system via any wall pad of the 8 zones?
I'm not even sure if the remote is a 'central' control point. I have not used their remote and it may just be a universal remote to control gear on the other end. On the other hand, it may act as a full system remote control to handle all zones... That would actually suprise me a bit. Getting a single remote that controls multiple zones starts getting into higher end gear which is often more specialized. I know Crestron, and I know I can do it - but it was thousands of dollars to get there.
6. If choosing Russound, would it be o.k. to wire directly to min. 2x speakers per zone as your suggest, and have a separate cat-5e cable from the wall pad location to the central location? When would one ever need a speaker cable run past such a wall pad within a zone?
You should follow the instructions with whatever you buy. Some wall pads have internal amplification and can power speakers right off the wall pad. Others just need a cat-5e cable at the wall pad. Depends on what you end up getting, but I run my speakers straight to the amplifier, and typically would follow that mentality.
7. RE power / amplification: I am not sure I fully understood that aspect. Continuing from the previous speaker numbers, what sort of power would I need for de facto up to 30 speakers for in a stereo sound arrangement (i.e. min. 2 speakers per zone)?
Not sure what you mean with 30 speakers. The Russound system only supports 2 speakers per zone, and the amps built into the system only support an 8 ohm load at fairly low volume. It's ideal for some nice background audio. If you have one zone with more than 2 speakers - like 8 or 16 or more, then you will need an external amplifier with enough power/channels to support your specific needs. Speakercraft, Niles, Sonance, and others all make multi-channel amplifiers designed for audio distribution. If you have a more specific area that needs a lot more speakers, then you will need to figure out exactly what you intend to do and amplify accordingly.
It sounds simpler than I thought, but I have a feeling that I am still misunderstanding in particular “sources” and “amplifiers”…
Sources are what you want to play across your home.
For example, in my home my audio (only) sources include AppleTV, XM Radio, and FM radio. We have a bunch of rooms available as destinations. I use external amplifiers for everything, but the Russound system has a built in 8 channel amplifier which can power up to 4 rooms with stereo sound (2 speakers per room, 8 speakers total). In my case, I have a bunch of 8 channel amplifiers which accomplish a very similar setup.
Should I be listening to XM radio in the kitchen, and my wife turns on XM radio in the master bathroom, and she changes the channel, then the channel changes in the kitchen as well.
If I didn't want that to happen, I would need to put a second XM radio onto my system so that I would have available to me 2 XM radios, AppleTV, and my FM tuner.
XM radio - AppleTV - FM tuner - Those are the sources.
Amplification matters a great deal in the fact that you need enough amplification for the speakers which will be used. Generally a room is a zone, and any single room will have a stereo speaker pair in it. If you go with that as your standard, then the Russound has enough amplification built in to support 4 rooms (8 speakers) per unit, and up to two units may be utilized with up to four sources total.
It's not a very complex setup that you asked for, but you could certainly ask for a lot more which will significantly change what you are required to purchase.