Less succinct answer:
The 555PS retains the effortless, smooth musical flow of the CDS3 but gives it the drive / fun factor / balls that the CD555 and CDX2 both possess but which the XPS powered CDS3 lacks.
We preferred the CDX2/555PS to the normal CDS3 until we tried a 555PS with the CDS3, at which point there was no going back.
I had recapped XPS2 with my CDS3. The 555PS sounded better with the CDS3 but at the time my main source was the NDS so I had the 555PS with that and the XPS2 with the CDS3.
Just out of interest is your 555 made by Naim or an alternative manufacturer? If it is made by someone else it is likely to sound slightly different to the Naim model.
I used to use 555PS (non DR) with my CDS3 and it was great. These days the 555 has been ‘promoted’ for service with CD555 as part of a 555 pairing and I use CDS3 with an Olive XPS. Very good indeed but not quite at the level it was with the 555.