I'm using a LS100 with my active ATCs. I had a SS pre before with balanced connections between the two. It sounded fine but I always wanted to try valves. As you probably know, ATCs can be really clinical albeit incredibly accurate.
The LS100 really has improved things considerably. However I cannot really comment with any authority as its my first valve pre! What I can say is the phono stage is excellent, the headphone out is as good as any headphone stage I've heard and will comfortably drive difficult loads and the build quality is as good as the Chord Choral system I used to have.
It is a big piece of metal - beware the depth (height and width are standard) and the remote is a bit basic (I couldn't care less about that).
If you are local to Wiltshire, please feel free to pop in & have a listen. Alternatively, speak to Jack (BD Audio) who I got it through. His service is excellent and he is very helpful.