Replacement CD Player?

The biggest difference is in the audio part. But you have a cheap alternative. Since your Meridian 561 DSC has an optical digital input you only need a pretty basic cd player, as long as it has an optical output (or coax if you are willing to put up with a - simple - coaxial to optical converter). The part of the cd player that is responsible for the sound quality is essentially the DAC (digital to analogue converter), and you already have that in your Meridian 561 DSC. In fact, you also have optical inputs on the speakers  - I am not quite sure how these units combine. What is the purpose of the 561 DSC?

The 508-24 was one of the best in its era. I'm not sure that $1500 would capture what the Meridian had to offer. The Hegel cd-only players are at the top of the game today, but expensive. Rega, Arcam, and Cambridge Audio, as well as Marantz and Yamaha are making very fine players these days.

Two CD only players that I would consider in your price range (new).

Exposure 2010

http://shop.generubinaudio.com/Exposure-2010S2-CD-Player-132698575963.htm

Musical Fidelity M3

https://www.musicdirect.com/disc-player/musical-fidelity-m3scd-cd-player-dac

http://www.audioadvisor.com/prodinfo.asp?number=MFM3SCD


Rich 

However, the OP does not really need a complete CD player. A normal CD player essentially consists of two parts, the drive/laser assembly that produces a robust digital signal (for years Meridian just like many other high end manufacturers used bog standard Philips asemblies), and a DAC that converts that digital signal into an analogue signal.
Thanks to robust error correction, the digital signal is a bit perfect copy of the original recorded file (see here for an explanation of digital error correction: http://abcnews.go.com/Technology/story?id=119305&page=1) So whatever your player, and as long as you have not totally destroyed the disc, the digital file will be the same bit perfect file. Sonic differences (though quite small) only emerge afterwards, when the digital file is converted into an analogue stream. Since the OP already has a digital to analogue converter in his pre amp/processor all he needs is a player with a digital output (preferably optical in his case). Whatever player he uses, as long as he uses the digital output, there will be no sonic differences. So he might as well use that old Sony Playstation 3, or whatever he has or buys cheaply. The only two things to consider are the mechanical noise level of the drive (that proverbial Playstation is pretty noisy) and the convenience of the user interface. But make sure it has an optical output.