Now we're cooking! That's as I explained pretty much what I do.. Sonos for the whole house PWD/Bridge for the big system... Best of all worlds, of course if Sonos would just do Hi-Res it would make things much simpler..
And sincerely not to be rude or condescending if you'd explained it that way in your original post we'd have saved many electrons :-)
I really like the W4S server it looks nice, the W4S guys build great stuff at great prices and are very accessible. Of course I'd like it even better if they made a version that excludes the Hard Drive and the CD Rom.. I much prefer to be able to use the ripping software of my choice, and I don't like being limited to the size and type of HD they use in the unit. I also wonder how you edit metadata on the W4S system, trust me you Will have to edit metadata. I'd also check with the W4S folks as to how you'll be able to sync the files that the W4S server creates with iTunes. I didn't see ALAC files listed as an option and iTunes doesn't support Flac natively, you could use Aiff files, but Sonos doesn't work as well with Aiff as it does the others. Since the W4S server uses basically an internal NAS I wonder if you can create a Share and point iTunes and Sonos at it??? .
I still think that if the W4S unit could pull your music files off a NAS that it would be a simpler setup ( I need to call them and ask) then you'd have easier access to all your music on both the Sonos system and the W4S system, in addition to being able to pick the size and type of drives. Thats' how I have my PWD/Bridge and Sonos setup.. I rip from my Mac straight to the NAS and both units can access the files, with no computer needing to be on. I have my music backed up on the NAS as a Raid 1 mirror and I use a second USB drive as redundant backup.. always remember back up ,back up ,back up
As for a Mac Mini your right, if your trying to keep it clean and simple. I also think the W4S is a way nicer unit and will in all likely hood crush that Xiva unit sonically.The other unit to consider is the Bryston file player it sounds great and is stone cold reliable, although it doesn't have the flexibility of the W4S as regards internet radio and other music services.. I love Pandora ..
Just keep in mind with the Sonos the first unit needs to be hardwired to the router.. not a big deal it can be wireless from there out.. also if you can hardwire the Sonos Connect to the router ( if you have or can run a Cat wire to the main system) then you don't need a Bridge, that unit would be the first link of the system, I use a Connect as my first unit.
Anyway good luck, I've been doing the computer audio thing for a few years now (and audio in general as a hobby, passion and vocation for 30 plus years) and I'm always interested and excited about new , and better ways ( and other peoples) ideas and solutions and setups.. one can never know it all, and it's fun to learn.. one of the things that makes this passion so much fun!
And please feel free to email me if you ever have need of another "take" or have any other questions you may feel I could help out with.