I seem to remember doing some work on a Hartke bass amp a few years ago, if my memory serves me right it had a control on the front panel which allowed you to blend between a solid-state preamp and a tube preamp circuit, I do remember that the tube in it was a 12AX7, the rest of the amp was solid-state, to my ears the tube preamp sounded a bit warmer than the solid-state preamp.