I heard the A3 with the Mark Levinson 585 integrated. It was a really good pairing. I have heard the 509x and found it a little underpowered even with Harbeth. I have not heard it with the A3 which are likely harder to drive. I also felt the 509x was a cleaner sound than the Hegel integrateds and separates. The A3 seemed to like power based on how my single stereo Benchmark AHB2 sounded terrible with them.
I have owned 2 integrated amps that would be great with the A3. The CODA CSiB and the KRELL DUO XD 300i. I think a little warmth matches well with the A3.
I have heard the prior top end SimAudio gear, and I am not sure if it is a good sonic match with the A3.