Strange audio issue with K3/KPA1500 setup

[nx7u]

Working on remoting my K3/KPA1500 via RemoteRig.
If I drive the amp to 500 W or less, everything works great.
But on 40m (and to a lesser extent 30m), anything above 650W or so causes tremendous instability in K3 AGC and RF output.  Some troubleshooting revealed that the audio drive to the K3 was actually varying by up to 5 dB during high power TX on these bands (but not others).  It's as solid as a rock at 500 W and less.  I tried choking the MIC INPUT from the RemoteRig (15 turns of 3.5mm audio extension cable on a 2.4/1.4" Mix 31 ferrite) but that actually made things worse.  It would seem that the problem is somewhere inside the RemoteRig box...has anyone else seen anything like this?

[nx7u]

More on this:
All lines to/from the RRC are now choked with Mix 31 toroids, along with all interconnections between the K3 and the KPA1500, and on the output of the KPA1500.
The problem is even worse now, I can't use the amp on 40m at all because the AGC is so unstable.
I did try different software, and interestingly enough using MMTTY on DATA A-AFSK the problem goes away.  But (thanks to a tip) it turns out that if I set CONFIG:AFSK FIL=OFF (no DSP filtering of the transmitted RTTY signal), the problem reappears.  This must be a clue but not to me :-(

[sm2o]

You have to start in the other end, RFI is caused by bad balance in the antennas, put chokes on the antenna feeders to prevent RFt from coming in to the schack. Trying to filter the cables is hopeless if the RF is already in the schack

73 de mike