Slov,
1) You can indeed use the Benchmark as a pre-amp. There is a rear-mounted switch where you can choose between line-level output or variable output via front-mounted volume control.
2) The DAC1 seems to be pretty tolerant of its transports. I've used it on several sources, and I don't think I can credibly tell the difference between my cheap-ass DVD player, my not-at-all-cheap Simaudio Moon Equinox, and Apple Airport Express (with lossless-encoded files).
3) There is a front-panel switch where you can toggle between the S/PDIF, AES/EBU, and TosLink inputs, so you can have more than one source connected at once.