lumin direct to "main in" section on Yamahas/Luxmans/Accuphase?

It should be fine as the main in is basically a HT bypass that just turns the Yammy into an amp.  Just to be safe I’d start with the volume on both the Yamaha and the Lumin turned all the way down and go from there.  It’ll be interesting whether you prefer going direct vs. using the preamp in the integrated to be better.  Can be hit or miss. 

Electrically that's fine.  The only issue is gain, so long as you have a volume control in your Lumin and you use it, no problem.

I have heard that there can be an impedance mis-match from these streamer "preamps" direct into a power amp. A proper preamp in between is preferred.

And if it were me I would look for a preamp with at least a tube buffer or a full tube preamp to warm up the sound. Or just stick with the integrated pre in.

https://youtu.be/ArfvjucjdDI

You’ll be fine, I do it all the time. It’s the man’s way to go. The first time you listen, it’s like lighten your hair on fire.

I ran a D2 directly to an Audio GD Balanced power amp and a Denafrips Thallo power amp with zero problems. Worked excellently with both and should work well with your Yamaha.