Don't know if you would be interested but this article talks about a modification that can be done to a CS600 to allow it to use the same 12AU7 and 12BH7 tubes used in the CS600X. I had come across it when I was deciding between the 600 and the 600X.
"If you own an original CS-600 you need not fret, as the Leben HiFi Company has developed a relatively simple modification that can be performed by a qualified technician near you, so that your CS-600 can use the same type of vacuum tubes used in the first stage of amplification as the new CS-600X"