I hate to advocate for "dead" products, but the Logitech Squeezebox solution works great for nearly everything you describe. You don't actually need to purchase any of the Logitech Squeezeboxes. You just run their LMS software (free for download) on your server and run one of several streaming apps on your iPad or iPhone. Personally, I really like iPeng as that app - there's a version for the iPad and the iPhone. There's anothe iPad only app called SqueezePad that also works quite well. These apps can not only control a Squeezebox, but also stream to itself for playback or control another iPad/iphone used as playback. It can definitely stream flac - even at hirez. Not sure about dsd.
Of course, to get the best audio, you'll want an external DAC as you describe and there are solutions for that.
While Logitech killed the Squeezebox (a majorly dumb move on their part), the technology isn't going away.