A friend had Mani-2s with a McIntosh MA-6900. His power guard lights would come on more often than he cared for. He listens at higher volumes than many people do, but it still shouldn't have gone into power guard that often IMO. I think it's due to Mac's use of autoformers. Something about keeping the same wattage regardless of impedance.
The Manis need a ton of power behind them. When you've got that and proper room placement/setup, they're one of the best speakers regardless of price. Anything short of that, and they're a very difficult speaker to live with.
I'd look at something like a Bryston 4B SST as a minimum with them IMO.