As far as the M-900U running hot, I would be shocked if it didn't. For an amp that's putting out 150 watts/ch. into 8 ohms to be able to put out 1200 watts/ch. into 1 ohms it is a very high current design and thus will run hot. Looking at the chassis on the Luxman site, the heat sinks though heavy duty, are encased inside a thick aluminum shell. So the heat being dissipated by the heat sinks is trapped in the amp's casement and thus makes the chassis hot.
The good news is that Luxman must know that their amp. will be on the hot side and therefore have designed the amp's components to be able to work appropriately at that greater temperature. My amp runs on the warmer side, making my listening room comfy in the winter months. As for the summer, thank God for air conditioning.😛