I know we're talking pre-amps here, but...
I like the idea of the trim controls on Decware's Torii MkIII amplifier:
"Trim control on each channel so you can adjust the tightness of the bass perfectly for any given loudspeaker. It does this by putting the voice coil of your loudspeaker in parallel with one of the resistors inside the amp so that the loudspeakers fluctuating impedance can tell the amp to put out more or less power. It does this in real time, nearly at the speed of light.
Trim control on each channel for the treble. This allows you to adjust the top end this amp offers so that virtually any loudspeaker can sound right, be it a Lowther full range driver or your favorite hi-fi speaker."
And then on the Mini Torii amp:
"Adjustable sound - designed with high efficiency single drivers as a likely paring by many customers, a defeatable treble control was incorporated that works perfectly to take any hotness out of the top end, be it the speaker or recording."
I've never heard either of these amps, but like the idea of being able to make these kind of finer adjustments, which is about all I ever feel the desire to make, and at the amplifier level, to boot.