Prior to buying my CD6004 I seriously considered buying a CD63 KI Sig. They were always excellent performers back in the day & I'd often hankered after one. However the thought of replacing one 15 year old CDP, which had started to misbehave, with another 15 year old CDP, which could well start to misbehave, steered me towards the new kit.
Anyway having auditioned the CD6004 shortly after listening to a friends CD63 KI Sig I definately felt that the CD6004 was the better machine. I know, I know, our "aural memory" is vanishingly short & the two auditions were conducted using different amps & speakers etc but the CD6004 is really a superbly musical CDP that really stood out over the other machines I had listened to.
I also tested it in my system against my outgoing Sony CDP XE900E (a 5* winner back in its day) and the difference was night and day, the CD6004 was a much more accomplished player.
Remember also that these machines are getting a bit long in the tooth & whilst the components to service & upgrade them is reasonably easy to come by, can you really be bothered?