Your TT and arm are compatible with a Lyra Helikon, if you want one. That is a very nice cartridge. I don't know how good the Cary phono stages are, but most tube units will not have enough gain for the Helikon. I would take a look at good phono step-up device that would allow you to retain the use of your Phono stage in the Cary, since you already have the RIAA section and some gain. A MC Step-up Transformer will allow you to load and step-up the output of the Lyra, and will plug right in to the MM phono section on your Cary. There are a number of makers of step-up trannies, like Denon, Cotter, Shelter, Ortofon, Audio-Technica. Or you could use a "head amp" which does the same thing, only amplifies instead of using a transformer. I see no reason to obsolete your phono stage and go to an outboard one. A step-up tranny or head amp will do. You will want about an extra 30db gain from the step-up device that you get for the Helikon.
From a practical perspective, I generally do not recommend getting a cartridge that is as costly as the TT you put it on, unless you already cannot benefit any further from upgrading in the TT area. In other words, I think that upgrading to a TNT would be more sonically beneficial than getting a Helikon SL on your MkIV. However if you have no intention of upgrading the TT any further, then by all means get whichever cart you want on it.
From a practical perspective, I generally do not recommend getting a cartridge that is as costly as the TT you put it on, unless you already cannot benefit any further from upgrading in the TT area. In other words, I think that upgrading to a TNT would be more sonically beneficial than getting a Helikon SL on your MkIV. However if you have no intention of upgrading the TT any further, then by all means get whichever cart you want on it.