In response to your question, I've never known bryston to be known for brightness. However Thiel's on the other hand are known to be bright particularly in certain applications.
Having said that, ARC equipment mates well with each other, so from that perspective the 150.2 would definitely make for a better combination.
I'm saying that also because I have the 150.2 and have had the LS-2 mkII and brightness was not an issue with either of course I had different speakers and components.