Please recommend a good Tube preamp.

If budget is of no concern:-
CAT SL1 Ultimate Mk2
First Sound Presence Deluxe
Lamm LL2

A lot of good suggestions above, and a few interesting items that I have not yet heard (e.g., First Sound Presence Delux). Not knowing what sound/change you are going for, I would say that everyone's suggestion is merely a recommendation to AUDITION these items before buying, unless you enjoy the game of trading items.

Here are my suggestions on items to consider. Some are pretty hard to come by, but if you are game, they are worthy contenders. To me, these have that something extra, hard to describe, that make music sound vibrant and harmonically rich. They are:

1. Audionote (u.k.) (Anying from the relatively modest M-3 through crazy-expensive M-10)

2. Emotive Audio (Sira or the quite expensive but amazing Epifania)

3. Deja Vu linestage or preamp (something made locally in the D.C. metropolitan area by Deja Vu Audio, a McLean, Va. dealer).

how could i foget audionote? also a tremendous preamp!

Aesthetix Calypso.

Try the Tron Syren (www.tron-electric.com). Tron amps are hand made by Graham Tricker (and only him!) in the UK. His tube amps are of the highest quality regardless of price. However, as they are made in tiny numbers, most people haven't heard of them let alone heard one in the flesh. They are designed for the ultimate in performance, regardless of price and use bespoke components and specially designed circuits with short signal paths. I have just bought his own demo Meteor preamp (the Syren predecessor) and I have heard the Syren at length. It sounds like music with all the grain removed -remarkable. If you're in the US, he is prepared to sell you one on approval, although I doubt you'll want to return it.
Alternatively, you could buy from his website his second hand Nucleus line preamp and Nucleus phono stage (circa 1999) which is also excellent.