I run 509x with S3MK1, same 88db/4 ohm rating. At times I wonder what more current would sound like and at times I laugh out loud to myself just how good it sounds.
FWIW the demo of 509x and A3 was all the talk of the FL audio show a few years back.
Also FWIW used 509x is plentiful right now right around $6K, a bargain for sure.
Enjoy!