You got me! It's interesting to me that lowering that resistor to Bass 3 from 4K7 to 1K5 has increased bass only slightly - when turned fully open there is more bass but where I have bass set it's not much different.
What is more apparent is the reduction in TS-type mids, which is why it sounds so much more open and frankly better to me. With lots of playing I'm finding I'm still running the bass pot up around 3.00. But because the mids are reduced I'm finding I don't have to run the highs so high. So I am turning treble to around 2.00 instead of 3.00.
So to appease my need to experiment I might change the bass pot to a C50K. The sound is as good as I think I'm going to get from this circuit - it's actually becoming my favourite overdrive because it can do most of what the Majestic/Caliber 45 pedals can do but has a very slightly fuller tone.