Hi Adam,
Yes. Be aware the Separate button on the remote needs a long-hold to activate it.
Only if the Denon is also your source selector. Anything connected to the Luxman inputs will not go to the Denon. To use a sub for anything on the Luxman you'll need to use the Luxman Pre outs to your sub.
Preouts from Denon front channels to "main in" on 509X for home theater where Luxman powers front channels.
Yes. Be aware the Separate button on the remote needs a long-hold to activate it.
but wouldn't my sub preouts on Denon supply a signal to both subwoofers in home theater and 2 channel listening?
Only if the Denon is also your source selector. Anything connected to the Luxman inputs will not go to the Denon. To use a sub for anything on the Luxman you'll need to use the Luxman Pre outs to your sub.