Dear Thuchan, Do you have some general criteria for SUT's use?
Should they be used by all LOMC or is the output not relevant?
An SUT (Some call them Head Amps) are needed if your phono stage doesn't have enough gain to deal with a low output cartridge. For example, if you have a phono preamp that supports MM only and you have a LOMC, you would need an SUT because a lot more gain is needed for that type of cart.
This particular case is interesting because the OP has a phono pre that should support a LOMC, but in reality the lower the voltage, the higher the quality of phono stage is needed for comparable SQ. Especially since his phono pre uses tubes. With a cart output of .3, you're asking a lot from the phono stage. It will be pushed right to its limit.
In my opinion, its 50/50. Try the Pro Ject first, by itself, and if it sounds OK that's all you need. If you get some noise or distortion, then it would be wise to switch back to MM and use a SUT.