I don't use a preamp, just the digital volume control on my Squeezebox. When the volume is down, all is fine. The ringing starts at about mid volume and gets worse as the volume is raised. It is a very high frequency ring heard through the speakers. The amp is centered between the speakers in front of my rack, there is about 3' on each side of the amp to the speakers. I don't have the 2' height above the amp in my rack. The top glass shelf is only about 8" from the top of the amp.
The 3205 only puts out 60wpc and has been the largest tube amp I have owner thus far. I guess I'm just not used to the heat a larger amp puts out.
Oh yeah, as for tubes, the KT-88's are Gold Lion reissue while the 6BQ7 drivers are stock. Maybe it's time to replace the driver tubes?