I have heard both the 845 and 50a. In fact, a pair of 50a's (from Sounds Real) spent three weeks in my system. I absolutely love the 50a, best midrange I have ever heard. Very good bass control for such a low powered amp. Amazing soundstage both in terms of width and especially depth. 3-d imaging. Very natural musical sound. Pure joy to listen to.
After agonizing over the decision, I did not purchase the amps. I love them and may end up with them at some point in time. However, they were just not quite up to powering my Aerial Model 9s, which require a ton of juice (i.e. 4 bass drivers). For the 50a's to really make sense, I would need to get new speakers and was not quite up for that. The fact that the 50a's came close to matching with the Aerials is a real testament to the amps.
As to reviews, there is a copy of one on the DH site.