Feedback Problems

I installed a LR BAGGS Anthem on a Martin D 18.

First of all, the sound is really great.

But I got big problems with a deep feedback.

I play in a acoustic duo, with 2 guitars. I often play the Bass-Parts,
so Low cut or turning the Bass frequenzies out, is no option.

When i knock on the top slighly the feedback starts verry fast.

I tried to reduce the vibration of the top in manny ways, and that makes it better.
I ve put hard foam in the guitar beetween the back and the top, that helps.

But without a good vibration top, there is no really good guitar sound.

WE play the guitars through the mixer, the Eq is flat, and then to EV-ELX 15P PA.
There ist nothing else beetween it.

The balance beetween mic and element is in position where you can here that the piezo
is a little bit in front before the mic.
The feedback problem comes from the element.

The black mark on the element is turned to to the top. The bridge fits well.

How important is the fixing of the cable of the element inside the guitar ?
I only did you use one clamp. What is to be observed about fixing the cable of the element ?

Or is the combination of the anthem and a guitar with a verry googd vibration top, like the martin D 18
the problem ?

Please give me your advice what else i can try, to get that great sound on stage.

thanks a lot !!!