can I connect 2 sources directly to power amp (one XLR one RCA)

Never worked, but tried quite awhile back....use a 6in/6out switched patch box to cure the problem.  All rca, but that's what gender benders are for.

You won’t hurt anything,  but it might not work either.  With the Node being on all the time, it will most likely mess with the SQ of the Lumin.  You need something to switch from the Node to the Lumin. 

Can be done on a McIntosh  501 monoblock as they have a switch  which let's one input or the other hook up to it. I was looking at doing this with a McIntosh  tuner and a wadia  cd player for my bedroom. 

I have done this on a McIntosh MC462, which has a selector switch on the back. It worked. But when I unplugged the XLR cables, volume on the RCAs increased significantly. I’m unsure why?

Hello AudoCanada!  If the power amp has a switch on the back for choosing the input you want to use, it's probably OK to plug in both inputs. If there is no input switch, don't do it! The inputs will be wired in parallel and they will interfere with eacj other. NEVER flip input switchs with the power on! You are risking serious speaker damage, and possible amp damage.. Happy Listening.