Heard back from Technic. He provided two useful pieces of information:
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The ASD is actually a 15W peak or so amplifier. The OEM amp inputs are those 15W peak from the ASD outputs. 15W is about 8V peak.
The iDrive output is about 2.8V peak low level. The ASD inputs are those iDrive 2.8V outputs. So when you bypass the ASD what you are actually doing is taking the iDrive outputs and making them the new OEM amp inputs. You are lowering the OEM amp input from ~8V to ~2.8V. That will change the volume and bass impact of the OEM amp outputs.
Raise the Bass adjustment a bit to compensate for the loss of input level. Please note that your OEM amp also has DSP and time alignment in it, so you may hear one woofer sounding louder than the other. That has nothing to do with the harness.
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Your OEM amp takes 4 inputs and create 7 outputs -front, rear, center, woofers. if you use your balance and fader at your iDrive and the front and rear speakers sound normal to you then harness is passing the iDrive 4 outputs into your OEM amp 4 inputs correctly. Otherwise there is a problem with your harness.
If you notice that both your front and rear speakers sound normal then the harness is not the problem, it is the way the OEM amp creates the 7ch out of 4 inputs, in particular the woofer channels. All 7ch OEM HiFi amps (F3x, F1x, F0x, F2x) have DSP and time alignment. What that means is that your OEM amp is taking into consideration the input level to create and process the 7ch output level and time alignment. The bypass harness by design is changing that input level as described below. That is the cost of bypassing the ASD.
If you do not bypass the ASD then you will notice that the driver side woofer still outputs less than the passenger side. The same happens in all OEM 7ch HiFi amps, regardless of ASD. All of them. The level difference is not that apparent as bypassing the ASD but it is there, just listen closely without using the Balance or Fader adjustment. That is the OEM tuning of the OEM amp. If you replace the OEM amp with aftermarket amp (XD600/6, for example) then both woofers will sound at the same level -and much better than OEM- just because the DSP tuning will be gone with the OEM amp.
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I suspect things are working "correctly" per his description of how the HK amp /ASD work. I will reinstall the harness today and confirm the front/rear left/right speakers are working normally with the harness installed. If they are then the complete lack of bass in that woofer seems like its normal.