I've run the CPU at ~1.73V before. The main thing is it heats up fast when under load, even with water - it takes about a second to go from 20ºC to 30ºC, and that's probably what causes instability.
Sometimes running at lower volts increases stability due to there being less heat.
I like this 890FXA-UD7 board. It doesn't suffer vdroop like my M4AA79XTD-EVO board does. It also doesn't have a limit of 1.7V
The NB speed is stable 24/7 at 2.8GHz on this. It wouldn't even POST at that speed in the old board.
Haven't really played around with RAM timings yet since the board change (still at 6-8-6-18 1T)