The Sonic Frontiers products due to the fact that they are DC coupled, low output impedence (for a tube product) and fully balanced are excellent companions to many solid state amplifiers sharing these design parameters (w/ corresponding low input impedences). The higher you go up in the line the more compatible these tube pre amps are to solid state amps. They are well built with nice convience features. How ever they tend to be more neutral and less euphonic than many other tube preamps in IMHO.
