First, I must preface my response below by saying that I owned a SD MP-2, which was the predecessor of the MixPre. Some have said that it's basically the same preamp, others have said that it has an improved sound. I don't know which is correct, however...
My opinion of the SD MP-2 was that it was overly loose on the low end and lacked a tightness/detail of sound that I wanted. People turned me onto the MP-2 because it has a nice warmth, and I agree with that assessment, but I couldn't get past the looseness which, at times on my recordings, was on the verge of an almost distorted-ly dirty bass sound. Bottom line is that I didn't like the MP-2 at all.
Some have recommended the V2 or V3. Personally, I don't care for that combo either because it's just too bright for my tastes. The KM184s have a real sparkle on their own and the strength of the V2/V3 is detail and transparency in the mid and high frequencies...but too much of a good thing is, well IMHO just too much. So for my tastes, the grace sound is overkill when you match them with the KM184...that combo was almost shrill to my ears.
Personally, I think that Rob Clarks recordings are amongst the best 184 recordings on the net and he uses a W+ UA-5. Check them out on LMA.
So, I'm not sure if you're into the UA-5...they seem to be out of style lately. If not a W+mod UA-5, then I'd go for a pre that tends towards warmth while maintaining detail. So, I guess of the preamps that I'm familiar with, that to me would mean something along the lines of the Oade M148, MMP, Aerco MP-2, PSP2, PSP3 or, for all-in-one boxes, any of the Oade warm-ish mods or Busman classic mods.
My two cents anyway.