Can somebody tell me what is going on with my engl cab?

Hey all,

I've got an engl standard cab that works great in mono, but whenever I flip the switch to stereo and plug in a second amp (using the 16ohm plugs now instead of the 8 in mono), one of them will always start getting crazy high-pitched tube osculations (no matter which 2 amps are used). I've opened it up just to see if there were any obviously disconnected wires or anything but it looks fine.

My other cab (1960av) is also stereo and does not have this problem, but I prefer the engl one.

Any help?

Phasing issues. Need to use a pedal to stop it. Look at the Lehle A/B pedal.