The x255 is only "fair" in its D/A convertor; the 510 MD deck's DAC sounds more spacious, as though there is more stereo separation and depth. I have a nice A/B switch I can quickly rotate to flip back and forth. The x255 is *huge*, twice as tall and twice as deep as all my other gear. It's hard to figure out where to put it; it's the size of a medium-large desktop PC box. Loading the x255 with 200 CDs, and managing those CDs, is very time-consuming. I have about 15 gripes about the 255. I'm thinking about sending my old CDP70 to a good, official Sony service center to fix the eject stubbornness. That old CD player is really good and easy and sounds great. No optical output, but the analog output is really good. My ears are spoiled.
There is a *lot* of room for improvement in the X255. It's "shuffle" would be instantly fired if someone employed it as a DJ; it's not even an algorithm in any sense; it's just a sheer stupid random selector, for which "Disk 13, track 3; Disk 13 track 4, Disk 13 track 5" constitutes a "legitimate random series of songs".
The X255 does only a fair job at the main task I bought it for: making MD mixes. There is no level normalization. On the other side, the 510 MD deck, as good as it is, screws up the simple task of removing the excessive blank space between songs. So, Sony's solution here is not working correctly. I still have to go through each MD mix and clean up the automated trackmark insertion.
I recommend getting a simple Sony ES deck, not a jukebox.