I am new to AVA products but not high end audio. When my Audio Research amp blew a few months ago I decided to give the 600R and new FET Valve CF try. I'm very glad I did. I will offer this quick review. Whenever I introduce a new component into one of my two audio systems (once every decade or so) my sole measure of success is if it makes listening more enjoyable. In some cases the improvement has been small. In this case I have been blown away with both the quality and scale of improvement. I find myself rediscovering music I haven't played in years. They are driving my B&W Nautilus 802's which sound like different speakers. Improved dynamics, speed, openness, and black field come to mind. Extremely happy customer.