Any experiences with FM Acoustics Preamps

I'm considering the single-ended FM-155 to go along with the Symphonic-Line Kraft amp.

Coincidentally, I used an identical combo some years ago: a stereo Kraft with a 155 & later a 266. Both with excellent results, better than the RG3 & the tube one. The FM is hyper detailed and the Kraft is (reportedly) wide-bandwidth, so all is fine. I didn't find it cool -- but then my whole system isn't very cool anyway; it's on the warm side of neutral (i.e. 100Hz is higher than 10kHz). As a matter of fact, I slightly preferred the little 155 (but the reason is probably psychological).
Other pre's that did well with Symphonics are the old Goldmunds, and the CATs.

Arupg:

Why and what were your findings when you went from RG3 to FM and then transformer?

The FM was far better than my RG3 -- which was really good, all things considered. The over-driving wasn't a problem, the FM unit had been already used with a medium sensitivity amp & it sounded fine on mine (~1,2 v I think).
The reason I moved to a transformer is financial: I unfortunately needed to downsize my system and had to let the FM go. I tried a used Goldmund ("7") which was also very good on the Kraft -- but had to let that go too.

As to your conundrum between the RG3 & 155: the latest RG3 is reportedly better than previous versions, but I don;t really know it (my story goes back 3yrs). The point with the THEN rg3 was a slight instability in the high register -- which didn't exist with the FM.
Another advantage for the FM is the unit's size: it's tiny & light compared to the RG3 which is seriously bulky. Also I didn't have any remote for the FM (is there one now?).
Regards

I really don't think you would be risking much with the FM -- in my time it really sang with the Kraft.
The CAT also sang beautifully with the Kraft... admittedly, in tubes I preferred the CAT to the Erleuchtung (but the Erleuchtung phono is, perhaps, slightly better than the CAT's). Regards

Greenfiledhk, a bit late in the day

advise whether the SL Tube PreAmp Ref or RG3 MK4 will be best matched with the KRAFT 300

I have used both these pres (some yrs ago). Sorry to be pedantic, and pls believe me -- there is NO way I can answer the question: the two pre's sound slightly but definitely different.
As you'd expect, they are both an excellent electronic match to the Kraft.

able to drive proprly a pair of kharma mini exq.diamond 87,5 db/8ohm

The small monos (Resolution 111) should be more than enough for these speakers. Maybe even the Classic series 70W stereo could do the trick (I think it's cheaper than the mono's).

You shouldn't drive the speakers too hard, anyway --

Be warned that FM is many things -- but budget-priced is not one of them. They are VERY expensive!