For completeness, I quote the words of the Hauptwerk developers:
"If you load/route a stereo pipe in mono then Hauptwerk picks the loudest of its sample's two channels and plays just that one (to avoid phase cancellation through mixing the channels together), with its amplitude adjusted to match the average amplitude of the original two stereo channels.
Hence the result is about as convincing as it could feasibly be, but of course you're still inevitably discarding one of the channels so the result will be less realistic in that regard."