Anyone recommend a neutral tube amp?

I've been wanting to move to tubes for some time now. Lately I've changed most of my equipment. I now have a Rega Saturn CDP and some very revealing Opera Callas monitors. My amp is the Sim Moon 5.3 integrated. I like the neutral character of the amp, it simply takes on the characteristics of whatever is being played through it. Can anyone name a tube amp that is similarly neutral, that won't color the sound of the CDP?