TOPIC:

I've been doing some reading on gain structure but only roughly understand analogue systems so far.

I like the idea of DSP in my system but a couple of my amps (F4 & MOFO both DIY) are current gain ie no voltage gain, so need plenty of oomph from the preamp to get them going. My Mac audio source can do 2v which is the max for the standard Flex, but I also have a turntable with phono stage.  

I read that the max gain setting is 12db.  Assuming I can get 2v from each source how much voltage swing can I get out of the Flex?

The maximum output from the unbalanced Flex is 2 V rms.

If you had an input signal at the 2 V level and then applied gain within the Flex you'd just get digital clipping and distortion, not a larger amplitude output.