The baffles are so narrow it probably won't make a difference, but traditional convention has it that tweeters should be on the inside, to minimise confusing baffle reflections messing with the image.
By the way, Harbeth SHL5's do have air and sparkle, but maybe not so much with transformer coupled valve amps with high output impedance, as the bass then dominates too much - in my experience anyway The latest incarnation seems to have addressed this, but they're not so 'warm toned' now.
Tear down these walls; Cut the ties that held me
Crying out at the top of my voice; Tell me now if you can hear me