Hi,
I have an A400, a Yamaha AV amp AX863SE and also a new Marantz 6003 amp.
The A400 is still the best all round listen and seems to work well with any speakers i have thrown at it.
The newer Marantz sounds tighter than the A400 and has good stereo image, but i wouldn't say better. The AS500 is meant to be better than the PM6003 - When i contacted a dealer he told me it was a more airy lighter sound than the 6003, so i avoided it.
I would strongly recommend going in and listening to the AS500 with a decent set of new speakers like the MX2's or MA RX2's as i am finding most new Hi-Fi kit lacking compared to equipment like the older - Cyrus 1 amp, A400 amp, Wharefdale 420's speakers, Pioneer PD8700 CD (one of the first stable platters) which i could listen to for ever without any low or high problems.
I hope this helps