When is it time to consider adding a SUT?

Depending on whether you’re running the cart in balanced or unbalanced, the Ref 2 will give either 74 or 68 dB of gain(respectively), which SHOULD be plenty, with even a very low output cart. Especially given how low the noise level is with the Ref. The Benz LP-S is rated at .36 mV, which isn’t as low as many. Everything will depend on the rest of your system’s gain, speaker efficiency and your preferred SPLs.

You have plenty of gain for the Benz LPS. I use a Benz LPS with my ARC Reference Phono 3 with no gain issues.

Doesn’t the REF 3 already employ an SUT when running through their MC input. I think at times it’s overlooked, but most tube phono stages have built in SUT’s. Manley, Aesthetix, and as all who frequent A-gon know, Atma-Sphere, do not use an SUT. IME, 68 db should drive a .3mv Benz. Not an easy task for a tube stage without an SUT, but 68db should work, pending it's quiet enough for you. I drove a  Benz Ruby 3 with a Lamm LP2, and the 57.5 db gain was not quite enough.... Cheers -Don