This is going to sound like I don't believe you. Please don't take it that way; I don't doubt you heard what you heard...
What you're describing sounds very odd to me. I've heard a ton of Arcam stuff - not the rDAC - and never heard what you're describing. When I asked if the rDAC had "polite highs" I asked it because every Arcam piece I've ever heard sounds a bit mellow and easy on the ears in that region. Never heard an Arcam sound harsh or distorted in the highs.
To my ears, Arcam has always sounded well balanced and a bit laid back.
Are you sure something wasn't amiss with either the rest of the system or the rDAC itself? If it sounds that bad, maybe something's wrong with it? I'm not saying Arcam is perfect, but their gear has always sounded very good to my ears. It's never sounded out-classed by more expensive gear to me. Maybe not as good, but not to the extent you're describing.
Again, I haven't heard the rDAC. I can't say you're wrong or right. I'd be very surprised if there wasn't something off here.
I should also disclose that I'm not the biggest believer of burn in for stuff that doesn't move. Digital gear needs a good amount of warm up time, but burn in is another matter. Every time I turn off my Theta DAC or Rega Apollo for a while, it takes a good 2 hours or so of playing music before everything sounds right. Interestingly enough, the highs are always most effected by it. My Theta sounds shrill, harsh and distorted when it's cold. The Apollo does too, but not to the same extent. Maybe this was part of it?