Phase issue with magnetic - piezo blend

Hi all
I have a bass with graphtech piezo and a magnetic pickup. There's a volume dip when the pickups are blended. And I hear phase issues. The bass has an audere preamp, which was selected because of its active blend/buffer, which is Effective and convenient.

However, it doesn't magically solve what appears to be some kind of phase issue. I have switched polarity on the piezo input at the preamp. No change to my ear.

I am hoping of you know a way to allow me to mix these two pickups without a big change. Is there a simple thing I can add or do here?
OR a re-do: what is the 'right way' to work a magnetic and piezo pickup together on my bass- do I need to start over to make these play nice together?

 

thanks-
I don't think the audere is designed specifically for the piezo. But I 'seem to recall' it has an active blend feature somehow to manage buffering of mismatched pickups.
to reverse polarity I switched the wires at the input of the graphtech system. After hearing no change, I 'seem to recall' switching them back.
Each of 5 bridge saddles has wires to their input (jumpers if I recall correctly). Then run as a +/- wiring to preamp.