Okay, I looked at Nik's layout (the one you posted in the opening post) and compared to Ted Weber's TMB (non-EF86!) schematic and layout. Ted wouldn't have minded, he was an open sharing kind of guy and posted instructions on how ALL of his stuff worked, even his proprietary items). Keep in mind the EF86 model has one extra tube, which Nik numbers V0.
https://taweber.powweb.com/store/6m18tmb_schem.jpghttps://taweber.powweb.com/store/6m18tmb_layout.jpgSo, the tube breakdown on the TMB/EF86 is as follows....going across from EF86 to rectifier...
V0...EF86 pentode gain stage EF86 channel (duh.)
V1...12AX7
-V1A...
unused triode (this could be hooked to V2B in parallel for a little fatter sound)
-V1B...first gain stage TMB channel
V2...12AX7
-V2A...PI for the EF86 channel
-V2B...PI for the TMB channel
V3...12AX7
-V3A...second gain stage for TMB channel
-V3B...driver stage for TMB channel
V4...EL84
V5...EL84
V6...EZ81
Traditionally the 12A*7 closest to the power tubes is the PI, but with a real Model 1974x, that tube is the tremolo, to reuse as much of the original layout as possible, when the TMB was designed, they left the PI where it was (V2) and used the tremolo tube as the extra TMB gain stages. But I digress.
So, the EF86 channel uses the A half V2 for it's PI and the TMB uses the B half of V2 for it's PI. I don't see why you would NEED to enter both halves of the PI unless you wanted an FX loop that works on both channels.
Signal path of the EF86 channel...the signal goes from the input>>E86>>Tone control>>Volume>>PI (V2A).
Signal path of the TMB...input>>V1B >>Volume>>V3A>>V3B>>the Tone stack, it exits the Tone stack at the Treble pot >>Master Volume>>PI (V2B)
Best place to place Poweramp In within the EF86 channel is between the Volume pot's middle lug and the turretboard.
Best Place to place the Poweramp In within the TMB channel is either between the Treble and Master Volume pots (allowing you an input Volume control from whatever preamp you are using) OR between the Master Volume pot middle lug the the turret board.
If you need to enter both sides of the PI...then you would go into V2A between the EF86 Volume and turretboard
AND the TMB channel's Master Volume and turretboard.