I call it a preamp anyway, because it's easy. Technically it's more like a 'linestage with a hardwire-bypassable 4-way tone control and a cathode-follower line driver output stage'. It's not really a preamp, as there is no overall voltage gain when the tone controls are set 'flat' or bypassed. When the tone controls are bypassed, the only tube in the signal path is the 6SN7 cathode-follower in the center. The 4 tubes in the rear are type 0D3/VR150 voltage regulators. The power supply is outboard.
The tone controls are simple passive RC Baxandall filters (no negative-feedback loops). Each filter has approx. 10dB insertion loss, so each one is driven by its own voltage amp (the four 6SN7 tubes in front) for overall unity-gain when set flat. The 'Bass' & 'Low Mid' are both low pass filters. The 'High Mid' & 'Treble' are both high pass filters.
I try to balance 'purist' design with 'practical'. As I listen to a lot of older jazz recordings, some of which were poorly recorded (as are some of the newer recordings!), the 4-way tone controls are very useful. The 'Tone Bypass' switch puts it in 'purist' mode when listening to really good recordings. I know, a true purist linestage would be fully passive (with no line driver), but I like to have the system controls at my listening position. This rules out a passive linestage, as my line cables (to the power amps) are 20 feet long, making the cathode-follower line driver a necessity.