I've plugged the DVD into a different source on the pre amp......same results.What you should try is a different source component, such as the cassette deck. Connecting the same source into a different input of the preamp doesn't prove very much.
I made mention Arlie's of it sounding "fine" at lower level......not the case......just at low volumes more difficult to hear the static/thumps.That's different. Given that, I agree with Lloyd that the problem is upstream (ahead) of the volume control in the preamp. Meaning that it is being caused by either the CD/DVD player, the interconnect cable between it and the preamp, or by preamp circuitry (or tubes) that are "ahead" of the volume control.
I've included the preamp circuitry that is ahead of the volume control on that list of suspects, even though no symptoms appear when nothing is connected to the preamp inputs, because it is possible that a problem in that circuitry might not manifest itself under zero-signal conditions.
Regards,
-- Al