A quick look at a few living legends of custom preamp/mixers for your everyday sound system. There are a few features that seem to be included on all of these preamps: kill switches for treble, mid, bass, sub (or alternately, low mids), 10 band graph EQ, parametric equalizers for bass, mid, treble, etc.
Jah Tubbys Sound System (JTS):
Barracuda (retired since 2005/2006):
Also on the Jo Red website are extremely helpful tips and schematics to building your own DIY preamp.
And if anyone ever finds a copy of this, let me know: