Here is a 6 year old post from Audio Asylum that addresses your question:
"I use the single disc X555 as my reference player and the 5 disc 801 for just random playing around. The internal DAC's on both are fairly decent, but I use them through a SDP-9ES external DAC. the DAC in the SDP-9 is over 10 years newer and has much more detail than the aging internal ones.
Both have excellent CD tracking that will put most others to shame."
I think that DACs and cd players have improved substantially in the last 20 years and that you should be able to find something that makes a worthwhile upgrade in the changer.
"I use the single disc X555 as my reference player and the 5 disc 801 for just random playing around. The internal DAC's on both are fairly decent, but I use them through a SDP-9ES external DAC. the DAC in the SDP-9 is over 10 years newer and has much more detail than the aging internal ones.
Both have excellent CD tracking that will put most others to shame."
I think that DACs and cd players have improved substantially in the last 20 years and that you should be able to find something that makes a worthwhile upgrade in the changer.