Phono preamp or SUT for low output moving coil

You will probably not be surprised by my answer.  Measurements get us to a certain point, they do not absolve us of the responsibility, or perhaps even better, the necessity to listen and decide what sounds we prefer.  It is usually the case, but not always, that someone who knows how to listen will prefer equipment that both measures well and reproduces accurately.  Of course taste comes into it too, hence looking at audio broadly there are horns, dipoles (electrostatic and planar) tubes, various solid state devices, Class A, AB, D, MM, MC, digital and analog this and that and all manner of other equipment variables.  All technologies can measure well, all have distinct sonic signatures, and of course all have their adherents.  So it is not possible for me to answer your question in a definitive manner.  Subjectively though, to your question how accurate is accurate enough? The answer is accurate enough to please the principle listening audience.

Bill, I was just re-stating the existing bone of contention. In essence I agree with your position. Also, I took the Devil’s Advocate position in stating the argument for pinpoint RIAA accuracy, which is in my mind at least the idea that since the medium does have so many other sources of distortion and since there are undoubtedly RIAA errors in pre-emphasis incorporated into some of the LPs we love, especially the older ones, one might take the position that it is desirable to use a phono stage with the least possible error in de-emphasis, so as to avoid possibly confounding errors in pre-emphasis encoded in LPs. This is just fun and games to talk about and in reality I pay no attention at all to this stuff when listening to music, just like you and most others. Likewise, I do not believe that cartridge loading is such a black and mysterious art.

Since my Krell KRC preamplifier with MM/MC phono stage lost one channel

I’m using Aurorasound by Lundahl SUT . Quite good solution for my situation. 
while My PA Krell 300 is getting refurbished I’m playing records with Rotel RA-1570 which has just MM phono. The Koetsu black MC cartridge is sounding very nice. 
no idea when my Krell amplification will be back and usable, I’m looking around for new rather used amplifier and preamp. It’s a lot of these things out there I don’t really have ability to listen different pieces of equipment. More likely to depend on opinions, reviews. Just a little hint, Krell was for me a little to bright, to “aggressive”I would truly appreciate any advice. I had opportunity to listen Audio Research Amplification and it was quite great, can’t recall the models name.

I don't see why this is a question. There's no problem having enough gain with either tube or solid state with the cartridges mentioned. As long as the input of the phono section is properly designed its plug and play; no need for an SUT.

If you want to use an SUT you have more connections but it does allow you to use nearly any phono section that has 40-45dB of gain, like you would expect for a high output moving magnet cartridge.

FWIW, phono cartridges are balanced sources. SUTs can be configured to accept the signal in the balanced domain and send it out either single-ended or balanced. There are also fully balanced phono sections that are tube and also solid state. We developed the first of such devices back in the 1980s.

Balanced is nice because it can eliminate any sonic artifact brought on by the cable itself.

In my opinion, the Krell phono stages were OK as SS phono stages used to go back in their day, but these days there are both tube and SS phono stages that handily outperform them. Specifically, modern phono stages are in general more free of artifacts that I used to ascribe to transistors per se, a kind of clinical sheen over the music. Modern solid state units have overcome that issue.  Likewise, modern tube phono stages are no longer guilty of narrow bandwidth (droopy bass and soft highs), of which they once were accused, rightly or wrongly.  I'd recommend you (winalovski) consider a new phono stage, rather than to pour money into the Krell in order to repair it.  Of course, I hasten to emphasize that this is only my opinion, but it is based on lots of listening to a wide variety of phono stages.