My tube preamp is a Linear Tube Audio MZ2. Maybe tube rolling changes the sound, but I prefer a different tool: the five-band parametric equalizer in the RME ADI FS2 DAC. Sometimes I run it flat, and sometimes I run it with a boost in the lower midtones (250 - 500 HZ). The difference in sound is apparent. And the DAC should last longer than almost any preamp tube.