Is a separate phono pre really necessary?

One of the Primary reasons to use a SUT is to maintain the sound characteristic of your existing MM Phono Stage. I love my McIntosh mx110z's MM phono stage, thus I went for a SUT for my MC cartridge, boost enough, then into/thru the mx110z's MM Phono EQ, maintaining it's tube sound characteristics.

If you do not like the sound of a phono stage, like I disliked my McIntosh SS C28 Preamp's phono, then it is better to go to a separate phono stage up to line level, use the preamp for volume/balance/tone/mode as needed.

I actually found a $14. MM Phono that sounded darn good from Pyle. And, before I gave it to a friend, I used the optional built-in phono eq inside the AT120 tt over the McIntosh C28.

PASS is a desireable feature on a SUT so you can change headshells or different tonearms from MC gain to MM PASS, same wires to the preamp.

+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.