The Cary SLP-03 should work fine with the Cary CAD 120S. Cary came out with the mk II version of the 120S because other manufacturers preamps had lower gain levels, which were unsuitable for the original 120S. The only difference between the 120S and 120S mk II is that the 120S mk II has more gain.
All Cary preamps have plenty of gain, so the original 120S would probably mate better with any Cary preamp than the 120S mk II version would, as the extra gain in the mk II version would not be needed with the amount of gain that the Cary preamps put out..
All Cary preamps have plenty of gain, so the original 120S would probably mate better with any Cary preamp than the 120S mk II version would, as the extra gain in the mk II version would not be needed with the amount of gain that the Cary preamps put out..