Rafael:
You are correct in your assessment. If you already have a server (i.e.: the new PWD, SB device, or other), you will only be utilizing the NAS device for storage (i.e.: an external hard drive only linked to your network). Your server will do all of the work streaming your music files. As long as your server accepts flac, then you will be fine. It is true that iTunes does not support flac, but as you stated, that is a mute point since you will be relying on your existing music server to manage and stream your flac files.
You are correct in your assessment. If you already have a server (i.e.: the new PWD, SB device, or other), you will only be utilizing the NAS device for storage (i.e.: an external hard drive only linked to your network). Your server will do all of the work streaming your music files. As long as your server accepts flac, then you will be fine. It is true that iTunes does not support flac, but as you stated, that is a mute point since you will be relying on your existing music server to manage and stream your flac files.