Charles has a good point. Magnetic pickups are by nature inductors, and when coupled with the amplifier's input resistance, input decoupling cap, and stray capacitances of the wiring going into the amplifier, you've got yourself a resonant low-pass filter. And since input characteristics vary from amp to amp, you'll get a different sound from amp to amp. Stick a variable-resistance volume pedal in between, and it really schmears things up. If you put a buffering box like a Steel Driver between PSG and amplifier, that variation concern should pretty much go away, i.e., the inductive pickup is isolated from the amp and sees an unvarying load, and what comes out of the buffer box shouldn't vary anymore as a function of the amplifier input. Your steel still looks into the imperfect input of the buffer box, though, so flat response is still elusive (it just won't change any more if you stick with the same buffer box).
Like b0b said, treat you amplifier (cabinet and speaker included) as part of the overall instrument. Buffer, pedal, and effects, too. And like Charles said, turn the knobs to your heart's content. It's all part of achieving your own tonality, musical expression, and creativity.
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Bill * MSA Classic U12 7/4/wrist * www.rahul.net/thinker