Tube Preamp with adjustable gain

I agree with Mds who offered up the Calypso as fitting this bill. Though its adjustable gain is set to two choices and is performed internally with jumper pins (which is a piece of cake to change since the top cover is velcro'd into place). But best of all, this is a truly world class amp and opens up the ability to taylor the sound to your goals via its exceptional responsiveness to tube rolling. One of the best preamps you can get in my opinion. FWIW, I like the LS25 MK1 way more than the Mk2 version due to the fact I am not crazy about the tubes used in the 25MK2. But that is a matter of personal preference and flexibility (IMO).

The Gamut D3i I believe also has adjustable gain by input. I had one in my system for a while, but never worked with the gain adjustment aspects.

Supratek preamps also have adjustable gain.

The New Eastern Electric Avant preamp has adjustable gain.

some photos here at 6moons when the DAC was reviewed.
http://www.6moons.com/audioreviews/easternelectric/dac_2.html

Disclaimer: I am the North American distributor for Eastern Electric.

all Joule-Electra preamps have adjustable gain from 0 to 17 dB ( by change jumpers, extremely easy)

As mentioned above some of the earlier ARC preamps have adjustable gain. I used to own the LS-26 (now discontinued and superceded by the LS-27). The LS-26 is an excellent preamp and has three gain settings that you can select via the remote control. So no need to open up the case and mess with jumpers. You did not mention budget by the way. What's your upper limit?