It may not be up to the standard of the rest of your system, but that’s pretty much what a Schiit Saga does. Unity gain if you turn the stepped attenuator all the way up. There’s a solid state and tube model.
Hattor has a nice looking tube buffer meant to be used with their passive preamps. It might have some fixed gain though. I’m not sure if that can be defeated, bypassed or altered as an option.
i spent some time trying to find a preamp with little to no gain, even started a thread on it here, but eventually broadened my search. While I didn’t need any gain, getting a little hasn’t seemed to hurt anything.