Is a SUT needed?

A good quality SUT is IMO the best way to amplify the low output (<1 mv) of mc cartridges with adding any noise! My first transformer was a Verion P designed by Mitchell Cotter. This was back in 1977 and my mc cartridge was the GAS Sleeping Beauty Super Elliptical. I presently own 5 SUT's - but alas not the Verion (sold!).

The gain is adjustable in eight steps, with up to 75dB of gain at the maximum setting. Nine different cartridge loadings are provided.

http://www.theabsolutesound.com/articles/tas-196-aesthetix-rhea-signature-phonostage-1/

I doubt you need of a SUT with 75db gain. My cart is .24 mv output and my phono pre (JLTi) is 62db gain which has plenty of output. Check your settings if you haven't already. 

Do you have the XV-1 XV-1s or XV-1T.  It depends on the both the internal impedance and output voltage to determine which step up or whether an active stage would be better.  If the impedance is low enough, a quality SUT will sound better than the active stage in the Rhea.

@handymann
You might want to check the gain set up in your Theta - it is adjustable. Michael Fremer used 56db gain setting with the Dynavector XV1S when he tested the Theta, there is still another 20db available.

The use of an SUT is debatable - you might get a lower noise floor, but at the expense of other distortions that SUT's introduce such as phase anomalies and distortion, and another set of connections and leads that can only reduce transparency. I have many well regarded SUT's along with FET, Tube and Bipolar based MC step-ups/phono stages - SUT's (phase anomalies, slow and suck dynamics) & FETs (too coarse, grainy) belong in the department of last resorts.