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

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.  

The the OP as you travel on your audio journey do yourself a favor and try running the D2 USB out into a nice R2R DAC and into a tube integrated amp. You do not know what you are missing.

As a Lumin D2 owner (in the box up for sale) I can tell you that the internal DAC is a little harsh sounding with many amps and running it straight into a power amp is not going to sound the best.

But to each his own. I have gotten to the point where I cannot even listen to solid state anything anymore without getting very angry and smashing the speakers....😀

Just make sure you have the Lumin's Leedh volume control activated (variable output) and start on the lowest level setting to be safe. Using a good preamp sounds better to many listeners, but there should be no problem in using the Lumin's variable output into an amplifier directly.