"Icebreaker's stuff is OK, but the legs on the boxers stretch out while you wear them, so they are nice when I first put them on, but a bit loose by the end of the day. "
I would describe the Smartwool Microweight Boxer Briefs (the only merino underwear I've tried) in exactly the same words.
The best boxer briefs I've ever worn, and best by a long shot, are made by ExOfficio. They're not merino, but they're great.
Of course, YMMV.