Col, if you are doing your mix inside the DAW, do you have a convolution reverb? Those are extraordinarily good at emulating rooms, halls, theaters and the like. Think of them as 'sampled rooms'.

For stuff like this, try recording the instrument parts bone dry, then use a theater convolution reverb (or whatever size you want) and send the instruments with enough to make them sound live, and your vocal just a BIT drier...

I have had great success making small stage and theater projects sound like they WERE in a theater...