The preamp of my new guitar amp will have lots of knobs and lots of tubes. However, through bypass relays, it will be possible to cut the signal path down to a one-tube preamp with not much more than a level control, akin to early amplifiers like the Fender "tweed" models.
One key factor in shaping overdriven guitar tones, to me, is the capability to shape the spectrum of the tone (i.e., EQ) both before and after the distortion-producing gain stage(s). The spectrum of the tone going into the gain stage has a big effect on the character or "texture" of the distortion; changes such as turning up the bass will not necessarily produce more bass at the output (indeed, the result could well be more treble and high harmonics), but the nature of the distortion and the playing-feel will be greatly altered. Changing the EQ after the gain stage, on the other hand, will have the expected results on the final sound: turning up the bass will result in more bass being heard, etc..
Different guitar amps have historically placed their tone controls at different points in the signal path (e.g.: Fender: close to the input; Marshall: close to the output), and this is one big reason that different amps sound different from each other. Many guitarists, perhaps unwittingly in some cases, have discovered the importance of being able to EQ the tone in at least two places in the chain; but since most amps don't directly address this, they use EQ pedals or other frequency-selective boost pedals, ahead of either pedal- or amp- produced distortion, and then they use the amp tone controls to shape the final sound. Many of the popular distortion pedals distinguish themselves through the tonality they impart with built-in pre-distortion EQ. For maximum flexibility, my amp will provide two full sets of active EQ controls (bypassable), before and after the gain stages.
The usual tone-stacks used in Fender and Marshall amps provide bass, mid, and treble controls, but due to the design of the passive R-C circuits, it is not actually possible to generate a mid-frequency "hump", just more or less relative proportion of "mid scoop". A mid hump, aka formant, is a fundamental aspect of the "voice" of most musical instruments, including guitar. Most guitar-plus-amp systems exhibit a formant, often chiefly due to the frequency response anomalies of the speaker cabinet. But the only way to change such a formant is to swap cabinets. Twiddling knobs on most amps won't make much difference to the formant. Again, some guitarists employ frequency-selective pedals to produce or alter the formant shape of their systems. EQ pedals are an obvious approach, but also the common trick of using a wah pedal kept at a fixed setting, has been used by many guitarists over the years to produce a dramatic formant in their tone.
Because of the importance of formants, I am electing to use a 4-knob active EQ, in both the "pre" and "post" positions of the signal chain. This EQ provides fixed bass and treble controls, based on the "Baxandall" circuit, which enables both boost and cut of the fixed-frequency "shelves". For midrange, a third boost/cut level knob is provided, along with a frequency knob (initially, I'm looking at a 10:1 range, from 200 Hz to 2 kHz, though this may change with experimentation). There is no control to change the "Q"; i.e., this is a "semi-parametric" mid.
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