Doh, I retract my retraction so deleted it.
Here's the piece I find important.
Per Luxman's website, the 590 has:
Bipolar 3-parallel push-pull
But the 509 has:
Bipolar 4-parallel push-pull
Implying a lower output impedance.
Is this critical? No, but my experience is that amps are more sensitive to a speaker's impedance than we believe from the pure math. On the other hand, the 507 also has only 3, and I find it very capable, but .... my speakers have a higher minimum.
So, if you had the chance to listen to both, I'd 100% recommend you listen to what happens in the mid bass as a factor to consider. It may be zero, you may even like it.
Also, yes, the 590's actual power output for music listening will be the equal of the 509/507.
Best,
Erik