Best preamp for my Krell FPB 600

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

+1 @jmcgrogan2 synergy is everything!

I have a couple of Krell EV 600's. Love their sound and strength. I like a tube preamp, although no matter how much u spend, at least in my experience, you're always going to have "tube rush."  When my system is "at rest" my Aesthetix Signature preamp puts out around 5 db of sound over what is heard when muted. From my experience, your power amp will merely amplify what's being put in to it.  If you're in to classical music, or anything with a lot of very quiet passages, u may want a SS preamp. I just installed some RCA quad matched 12ax7's low noise tubes I bought from Vintage Tubes and still have a somewhat significant noise floor, but can only be heard when music isn't playing. It depends on your particular taste. To me, tube preamps, have more of a fuller and buttery sound. 

I have the Absolare tube preamp and I don't hear any tube noise when I place my ear to the speaker. I also don't get a buttery sound. I have heard the Aesthetix preamp in my system and I agree that it is a warmer sounding preamp. I did not like it in my system, but I love the Aesthetix Io phono stage which is what I use.

handymann, the absence or presence of the noise that you mention is not (necessarily) due to your tubes. I have one all tube preamp and a tube integrated and both are dead quiet even with they volume all the way up - of course with no music played. And in your case, changing tubes will probably not solve your issue since the "noise" is mostly due to the transformer design and the gain matching between the preamp and the amp. Tubes themselves, if relatively new and from a decent brand don't contribute much to the noise floor.