Best preamp for my Krell FPB 600

A good match is in the ear of the beholder.

Over the last 15+ years that I have been perusing audio forums, and 30+ years reading glossy rags, I now realize that sometimes I agree with a writer, and sometimes I disagree.
No big deal if someone's opinion is different than yours. That's why they make so many different flavors.
You pays your money, you makes your choices.
Trust your ears. If it sounds good to you, it is good.

The ML 38S is an excellent preamp and will sound fine in ANY system!

See, I cannot agree with this statement.
That is where synergy and subjectivity come into play.
Personally, I owned the ML 38S for a few weeks (over a decade ago), and I did not care for it at all. However, the gent that I bought it from loved it, and the gent that I sold it to loved it. So I would never say that it was bad gear, it just didn't suit my system/tastes at that point in time.

So it's all about system synergy and personal tastes. That is why I told the OP to just enjoy what he has if it sounds good to him. 
I did not mention my poor experience with the 38S, because that is irrelevant.

From my experiences, ALL gear can sound great with the right ancillary equipment. There is no one piece that sounds great with everything, just as there is no one piece that sounds terrible with everything.
It's all subjective, and system dependent.

Cheers,
John