I have heard Franks DVA 850 mono at his home and they are fabulous. They would sell for $10K in stores. TAS gave them a great write up.
Van Alstine DVA 850 Hybrid Monoblock Amplifier | The Absolute Sound
The SET 600's are great sounding mono's as well. I can't say that one is better than the other. I think that the 850's may have a little more air and the SET amps have a fuller midrange and a more romantic sound but not overly so.
These amps are a huge step up from early AVA designs. It brought AVA into the high end arena for a relatively budget price.
Both sound better than my Pass amp.