For the classical and Jazz the ribbon becomes a matter of taste, the rock however it might be a bit of a limiting factor.
S/PDIF will output like a standard voltage range yes. That range however can be adjusted *DOWN* depending on your hardware/software, it will still be in the range, but will output a smaller section of the range, if that makes sense. Most Line Level or Digital Interfaces work like this. You can usually take them to a quieter signal with no problem though.
Funny you should mention VCU, I am putting my wife and myself through it currently
In as far as using a headphone amp in front of your powered monitors... I hesitate to do that. I understand what you are trying to do, there are better passive ways to do it(I have a Presonus Central Station personally serving the same purpose). There are two things that concern me, one is impedance, one is overall levels. Your headphone amp will not be designed to only put out line level like your monitors are expecting, and thus it is easy to screw up a gain stage doing that, not to mention possibly damage the inputs on your monitors if taken to much into the extreme.
The impedance issue is a bit more tricky, especially since I would have to look it up to be sure it is an issue, though I suspect it probably is in this case. Basically(VERY simplified) when dealing with line level devices, the devices are built for a certain impedance on the input and output side, in order to keep the signal clean. However if you change the impedance, the result can be coloring the sound, depending on several factors. Again it is something I would have to look up based off your setup, equipment, cabling, and the actual impedances involved and expected. If you are happy with your sound, fine, I might suggest plugging in a decent pair of headphones and comparing to see how far off it might be.
NOTE: The next two paragraphs are strongly ancedotal, but I think you will find they probably holds true, at least in the professional world.
I can say that Hi-Fi and recording hardware tend to be two different categories. Hi-Fi in general when used typically refers to the consumer/some audiophile equipment. Also in general this is where you will get the -10 levels of consumer equipment. Recording and Professional equipment will output a +4 signal on line level, to strong for consumer equipment, and generally falls into its own class.
So in many instances you are using more professional, or at least prosumer equipment than HiFi, despite them both being intended for 'High Fidelity', of course how anyone can call it High Fidelity when listening to MP3s I really don't know these days
The RAID 1 is nice, and will likely give you good read speeds as an added bonus to the data security, allowing you to mix down more tracks if you ever need to. Don't try to record to many tracks at once onto it though
In as far as isolation is concerned, you can do it several ways. Obviously right now I am running ADAT to a seperate Preamp/DAC/ADC, but I have also run standard copper and line level as well on occasion. If a ground loop should develop, isolating the grounds between your system will be all you need to do to fix it. But truth be told you don't have enough that needs to be on a separate outlet yet, nothing in that list should be drawing even close to 20Amps.
LAU is Linux Audio Users mailing list.http://lau.linuxaudio.org/
The expertise level ranges from absolute beginner that might or might not know the difference between optical and copper, to professional. But in general you can get good linux specific advice on there, but beware it is a decent volume list(Meaning you will get a fair amount of email from it if you subscribe). Setting up Filters in your client software is a good idea, I have a gmail account I use for my email lists I subscribe to to make sure I don't have a problem with space or bandwidth, but I subscribe to a fair few as well.