Hi Bjorn,
a. Sound quality (SQ) will not be negatively affected by transfer of (EAC ripped) music files over Ethernet from PC in study to be played by XX on another computer (PC or laptop) in another location in the house.
True.
b. Transfer over hardwired Ethernet faster and more secure than wireless transfer, but either mode of transfer OK (and does not interfer with SQ).
True.
But note that when the PC playing the music (your laptop) is receiving the wireless directly, we can expect that this *does* influence sound. This is because of the high activity of the wireless receiver/transmitter. Besides that, it can really incur for things like hiccups.
Whether this is really the case, depends on the particular implementation.
c. Laptop usually has built-in sound inferior to internal PCI sound card in desktop PC (in my case M-Audio Delta 192), but if hooked up to external DAC, laptop SQ not inferior to PC hooked up to same external DAC (using the same USB or FW interface, Vista and same associated driver).
Not true, but only because more parameters are involved;
A. generally *everything* a laptop does, is worse in the first place, and quite unsolveable in the second.
B. FW connections do matter, hence the transmitters do. For a laptop it can be a pain to get it properly working to start with, and next it must be sounding good. More expensive to try etc. etc.
C. In a desktop you have possibilities to tweak the USB connections to the sense that usually more "hubs" are internally provided. With a laptop *maybe* the same, but if so less accessible. Furthermore the laptop depends more on USB so it is not easy (convenient) to cut out half of the connections.
Together with laptops being sluggish always, I would never use a laptop. Only if you have one ready at hand, and it does all the things you want in a decent / good sounding way, go for it. Never buy one explicitly for the task, because the chance is 95% that you will be sorry.
I know, this is a typicle means of a "NAS" if you like, which should not be confused with a 100% dedicated music PC (which I would not call a music server). This theoretically implies that you can have the storage anywhere, but still need that music PC. If you have two music rooms, of course it is logic to use something as a laptop for it, if it is about you listening only. But better would be two PCs like you'd be having two CD players without a PC.
Lastly, note that this repsonse is in the context of nitpicking the best sound out of everything *and* is about avoiding the chances of not being satisfied (up to glitches and all) while you can't do anything about it, but being annoyed.
2. If instead of using Ethernet/wireless, you copy music files onto removable hard drive (HD), both USB and Firewire acceptable interfaces.
As SeVeReD said, USB should be avoided when you have an USB DAC.
Then, although in the base you are right, I see no reason to do so, unless you don't have the ethernet connection. But then it would be tedious, and you certainly won't be copying the music you want to play on a per day basis, and instead you would want it on line all. In that case the "NAS" principle has become useless ...
Does this help ?
