Hi Mark,
Welcome to AoS. Lovely system - I'm sure that it gives you many hours of musical pleasure!
If I were you, I'd think long and hard before changing the CDS2 for something 'better' with valves... I owned a CDS2 for years, and it's a superb piece of kit - far better than the majority of CDPs produced these days. If you're looking to
genuinely upgrade from it, you'll need to spend quite a few grand, so I hope that your pockets are deep!
I can say from experience (as a valve 'nut' myself), that the majority of CDPs, using valves in the output stage, are a huge compromise, and that valves are introduced merely as a 'bandage', in order to tame the rather 'spikey' top-end of most modern CD players and induce some euphonic coloration, which may produce a pleasing effect in some systems.
My advice, unless you have at least £3k to spend on a new CDP, is to leave well alone and instead upgrade those horrible Electro-Harmonix valves inside your (lovely) GRAAF! Trust me, you can do so much better, and replacing them with something like some nice NOS Brimars/Teslas/Mullards, etc, depending on taste, will provide you with a much more significant sonic upgrade than you're likely to get by replacing the CDS2 with your average valve-based CDP.
Just my 2p worth, mate, but it's based on considerable experience. Anyway, whatever you do, enjoy the music!
Marco.