ruthieandjohn
Stumbling towards enlightenment
(Formerly known as kayandjohn.)
Just for the record, then you're not listening to the Touch, which is now your transport. The Fostex is both DAC and amp and is now your source. The Touch 5G has no audio line out and can't be amped that way.
I believe that if you use the Apple iPhone 5 dock shown here: http://store.apple.com/us/product/MF030ZM/A/iphone-5s-dock you can get line out. Though designed for the iPhone 5, the iPod Touch 5G fits it too.
I have the dock (and the iPod 5G, and the iPhone 5S), and there is an audio line out that is unaffected by iPhone 5 (or iPod touch 5G) volume control, and indeed behaves just like the similar-looking Apple dock for the old 30-pin connector used on the iPod classic and the iPhone 4. The Apple writeup also says it is a line out.