An 840C is like $1K, right? Man, there's a TON of options in that range...
I haven't read much about the 840C, but I definitely haven't read enough to put it at the front of pack in that price range. But if you've narrowed it down to that or the Benchmark, and everyone says it's better than the Benchmark, then it's an easy choice, no?
Anyways, if you haven't built any amps before, I don't think a Beta22 is your best bet for a beginner project.
Finally, a recabled HD650 isn't far from RS1 or AD2000...