Of course they are spec’ed different and sound different, one is a 120w-8ohm powerhouse because of the higher rails the other not. They have different + and - rail volts, different wattage, and different bias.
What I’m saying if you care to study both internals is, if the rails were the same voltage on both and both biased the same they would sound the same.
The only physical difference is the power transformers primary wingdings.
590axII
https://lh3.googleusercontent.com/proxy/WZvXKTDwgZaj_Voh4gbYz8okG-mRK6WwZ_swA_KbkAYW11n5zvdOZGBmzkVP...
509X
https://cdn.shopify.com/s/files/1/1040/6126/products/luxman-l-590axii-integrated-amplifier-luxman-2_714x700.jpg?v=1574670463
Cheers George