Assuming the PA is a normal full-range system, a line level signal (I assume "kind' was a typo for "line") from the computer to the PA mixer should sound the same as it does in regular studio speakers or headphones. Since I don't know exactly what you're hearing from the PA that you describe as "flat and dull" or how that compares to what you're hearing in the studio, I can't really be specific, I can only venture a guess or two..
- If the PA speakers are not full-range, then you might very well find their band-limited response (lack of highs and/or lows) "flat & dull" compared with studio monitors or studio-grade headphones
- If the signal you're pulling out of the computer comes from the main output in the MainStage mixer, then it should be the same as what you hear in the studio, but if you're pulling it out from an i/o plugin or a send or an Aux in a way that bypasses some of the FX in MS (amp sims, etc), then that might result in a different (flatter?) sound from the PA
- If you're sending a line level signal from the computer to a guitar/instrument input in the PA, then a level/impedance mismatch might result in a "flat & dull" sound—make sure to run a line-level output to a standard line-level input
>> ..is there a recommended way to rout the outgoing signal?
It depends on how the MS Concert is set up, the outputs on the interface, and the inputs on the PA mixer—the easiest way would be to simply run the main outs from the interface to the PA mixer (line out to line in, as above)..