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

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. 

I’d use the Lumin’s volume control as it is digital (Leedh) and doesn’t chop off any signal when playing at low volume levels.  I’m using the Lumin U1 Mini into an external DAC and then into a power amplifier with excellent results.