My response may go against all of the others but I will give it a try. I have been an audio buff for the better part of 15 years now. I have auditioned much gear, and already gon through the money=better phase of audio. Now I consider products to really excel if the price/performance ratio is outstanding.
The 506.20 is a sleeper product from Meridian and is truly a classic in my opinion. At the price used it is untouchable. I have owned it for years and have yet to find a CD player even close in performance.
The 508.20 has balanced outputs and is significantly more money. The balanced outputs are "created" using op-amps and are not truly balanced. Listening to it single ended I found it to have less bloom and sound drier than the 506.20. I would have to stretch to assume in balanced it sounds better, but personally I don't care. A player anywhere near this amount of money should excel no matter which outputs are chosen.
Buy the 506.20 or 24. They are great.