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Height Control: Low and High
The ear interprets a fairly narrow boost centering at 8 kHz as a sound source above ear level. Conversely, it hears a dip centering at 8 kHz as a sound source at or below ear level. Implementing this control might require active equalization or at least a reactive network that includes an inductor.
Dynamics Control:
Compressed and Explosive
I know of no better sonic compressor than a healthy amount of global feedback. (Actually, this control might be better labeled as the "Life Control.") Achieving the contrasting attribute might require positive feedback. Now positive feedback is definitely a dangerous thing, as it leads to oscillation and burnt-out voice coils. Moderating the positive feedback might require a negative feedback loop or a low gain tubes. (I once heard a hotrod Dynaco ST-70 modified by my friend, Lance, that used both positive and negative feedback. The sound was wildly exuberant, so much so that it was likened to the singer, Madonna, as many would love to spend a weekend with her, but none would want to spend a lifetime.)
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I have also found that the amount of idle current can influence the perceived dynamics of the sound, with a low current sounding insubstantial, but dynamic; and a high current sounding solid, but a bit constrained. Possibly, the mechanism lies in the low idle current's working at the bottom of the triode's curve where the increasing transconductance with positive going signals is more pronounced.
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