What phono stage are you presently using? It appears that the CT5 is line-level only.
And what percentage of the volume control's range do you typically find yourself setting it at?
With that information, together with the info you supplied on the other components, it should be possible to tell which version of the cartridge you are using.
I assume that "250 S" refers to the ET-250S, which is spec'd as putting out 250 watts into 8 ohms for a 1.85 volt input. The CT5 is spec'd as providing 20 db gain (that would be with the volume control at max), and the speakers were measured by John Atkinson of Stereophile as being about 83db/2.83V/1m/~8 ohms. Those numbers together with the gain of the phono stage and the range of volume control positions you typically use should make it pretty easy to determine if the cartridge is the 2.8 mv or 0.3 mv version.
Or are you saying that you've never used it in your system, and you presently have no phono stage? In which case I have no suggestions beyond what the others have said.