Uberschall Twin Jet - Bad Power Tube Socket?

[JFUberist]

I have a 4 month old Twin Jet that squeals when the amp run for 20-30 mins. I have replaced all pre amp and power tubes and the squeal returns. The only remedy that I can reproduce is removing the outer pair of power tubes. Any one else experience this problem or know what it could be?

I would send it to Bogner, but would have to pay shipping across the entire country and could be without for quite a while..

Thanks!