you don't mention what your source is. personally I have benefited the most by upgrading (or tweaking) the source. the A-S500 is a fine looking amp for sure, but I'm not sure if you're gonna benefit the most if your source is not up to scratch. if you're using a PC as a source consider upgrading the soundcard with the best you can afford; preferably one that can be connected directly to the amp so avoiding the usb port. if you're stuck with the usb maybe consider an external soundcard and/or a usb interface (good external DAC's are freaking expensive).
I've not tried many CD players, but I once bought an Emotiva ERC-1 - before I received the ERC-1 I was using a DacMagic connected via its usb cable to the PC. compared to the DacMagic the ERC-1 was way, way, WAY better. it was fun while it lasted; alas it developed a fault and had to return it to Emotiva's, and since it cost me a small fortune in shipping/duties I didn't bother with it anymore and got a refund. however, later I bought the Marantz's entry-level CD6003, which I also compared to the DacMagic; the CD6003 was absolute rubbish, and nothing like what the reviews made it to be, and returned to the shop the next day.
in brief... consider upgrading your source , if needed (not to be overlooked, imo). good luck