I had the A3s with the 509x for a while. It is a good pairing. I never felt the amplifier was wanting in any way. It has good bass, is dynamic and somewhat exciting (not boring at all) and has good sparkly highs. I used Nordost cables with this pairing, fwiw.
I also had in my room with the A3s the JRDG Continuum S2 integrated, and also Primaluna 150 w/ch monoblocks. These both were boring with the A3s.