I have an AVA Insight+ SL with Phono Stage and have been very happy with it. It simply gets out of the way of the music -- nothing is added.
That being said, if you are interested in a tube-based pre-amp, I have a Mapletree Headphone Amplifier, and it is by far the best I have heard. I suspect Lloyd's pre-amps are very good as well. He, like Frank at AVA, is very easy to talk to.
They will both happily answer any questions you have.