Is a separate phono pre really necessary?

+1 @beatlebum Had a perfectly good phono stage on my Plinius integrated that was adjustable from MM to MC, but when I went to the Sutherland Insight (and then added the LPS after getting a nice deal on a used one) the improvement was immediate and noticeable. 20/20 is great too, but a good bit more if you go for the LPS, which on that model is in an external box. Is it worth it? My dealer told me I would be thrilled with either one, so I kept the difference, but coming to think of it, there were no used 20/20s out there, so it made my decision easier, saving about 50%, $1000. And Ron Sutherland is a great guy - guided me through the LPS upgrade process himself on the Insight.

Is the transformer you are using made specifically for this cartridge?  If so, then you may receive little benefit from the additon of a phono state, which in fact will create redundancy with the stage already in the rogue.   Make sure you have the right loading, right cables, right stylus setup, and all before you spend money on a separate phono preamp.  If the Roque's phono section is good, then at most you should be looking to add a headamp, not a full phono preamp.  

If you insist on having a separate preamp, perhaps you should look at Rogue's own, which has variable loading options including a custom setting if I recall correctly.

I’m going to say that the designer of a built-in phono stage has the goal of producing an input that the broadest range of customers will find “pleasant.” The phono stage in my Rogue Sphinx is very pleasant. When I started looking for a separate, I started asking what I wanted to hear. It’s a pretty fine line and so subjective. I ended up happy with the PS Audio Stellar. They occasionally have sales of about 30% off and their trade-ins are generous.

I had the RP-5 and used its internal phono stage with Hana EL cartridge. I was not impressed. I switched to MoFi UltraPhono and it was slightly better but still not what I was looking for (exciting sound but kind of flat in the soundstage department) so I returned it and bought a Sutherland KC Vibe MkIi. Very happy with it. The Hana cartridge calls for >400ohn loading and the Sutherland is perfect at 475. A very notable step up from what Rogue includes in the RP-5  

The Vibe MkII loading options include 100, 200, 475, 1k or 47kOhms. 
It’s quiet, resolving and very natural sounding. Build quality is top notch.  
I wouldn’t hesitate to recommend it. 

You can’t go wrong with Sutherland. I started with his KC vibe, then the 20/20 LPS and then the Duo, each incrementally lowering the noise floor increasing the dynamics.