His I'm trying to re-amp a guitar that was Di's to an external amp. I've done it before so I know it works but for some reason I can't get it work can anyone give me a clue as to the obvious silly thing I'm missing
 
I'm using a Presonus audiobox 1818vsl.  I select the output of the DI guitar track in sonar to audiobox driver output 5 which is the output I have the balanced mic cable going into the daccapo reamp box.  I take the output of the reamp box into the input of the guitar amp.  Surely I should hear an input on the guitar amp at this point, cos I'm not?
 
If I was getting an input to the amp I normally set up another track in sonar with the input source for the mic that is on the guitar amp but the problem is I'm not getting any output from the audiobox / input to the amp.  I'm doing something obviously wrong but can't think what

does your Audiobox have any routing software ?
i know my Focusrite 18I8 has a software called mix control. this lets me route different mixes, for monitoring, headphones, mixing, daw tracking, exc.
 
also make sure the obvious is going on "make sure input echo is engaged" so you can hear the signal.
though input echo wouldn't fail to send the signal, just make sure on your input that its on.
also double check all routing in sonar itself.
also check the input / output on the re amp.
ive put them in backwards before. easy mistake.

Can you verify that the reamping box, the amp and the cables are all working? The track is definitely making sound in Sonar? E.g. if you switch the outputs back to main L/R you can hear it? 

You can trouble shoot your signal path further - if the output of the audiobox in question is qtr inch - you can plug a set of headphones in to see if signal is hitting it - it will most likely be very loud so dont wear the headphones.