I'm a huge fan of the Elementary and Detective, both of which are based on the HiWatt DR-103 preamp. The HiWatt tone stack is an unusual one, and it's impossible to get it completely flat... so ditto with the Elementary and Detective. But following Senior Sushi's guidance, I used a white noise generator and Audacity's Plot Spectrum Tool I found that flattest settings that I could.
First is with bass/mid/treble at 11-5-9 (using clock face terminology). 5 means mids fully clockwise...dimed...turned all the way up. Pretty flat until 1K, with a roughly 2 db boost above that.
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Next is 12-5-9. You can see that boosting the bass just a bit has the effect of scooping the mids by roughly 2 2b because the highs remain boosted.
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Last is 12-5-8. Highs are slightly reduced.
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Some takeaways... 1. The mids will be scooped unless the mid control is at 5. 2. The treble control is very powerful. - anything over 8 boosts the treble. 3. I have too much time on my hands.