Good catch, EddieTavares! There was amissing GND-connection in my schematic, please find a new version attached (input and output resistors were not grounded). I also rearranged the circuitry around the second part of 4558 to improve readability.
I also found another schematic of the BB-preamp, which is dated april 2006. Seems like the Mesa is the clone here (they were launched late 2012, iirc).
Only major difference is the resistor arrangement around the second opamp. I desoldered some resistors and continuity-checked and am quite sure that my version is correct. Also, the flus drive has two clipping diodes in each direction.
And, if you compare BB preamps schemtic from 2006 with the Flux drive, the circuit between the 4558 and TL072 has the C-R-C-R arrangement which I believed was wrong in my schematic, but it's just like the original minus the pot/wiper - 100k in parallel to 47k gives 31.97k, plus the 1k from BB schematic ~ 32.97k, so 33k is pretty close
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