I would have to check with Emotiva to see what the internal gain level is set at compared to the others. Pre amp stage in all the amps.
My Nexus 7 and a certain crappy interconnect cable I have is a good test for hiss/hum and gain levels. On the Mini-X, it is very low. The noise floor doesn't creep up until you pass 12 o clock. It is practically perfect for the HE-500.
The Lepai direct without tone control is very low as well. Once tone control is engaged, the noise floor increases and becomes noticable. The benefits of tone adjustment outweigh the cons for me. I was lucky to get a used bare wire cable for a good price, but if you are paying ~$100 for a Lepai 2020a, cable, and 5A power supply, you might as well get the Emotiva for $169 instead. The Lepai is like 90% of the sound of the mini-X without tone control.
The Lepai 200W is just too high, too powerful. Needs resistors. Could be awesome if it has a shared ground and a resistor adapter in between.
The SMSL 50Wx2 sounds better than the Lepai, and just about as good as the Mini, but has a higher noise floor and some hiss at all levels. This would also need resistors. I need to find out if it uses a shared ground as well. If this is shared ground + $13 resistor adapter, it would be a much smaller mini alternative.