Honest1: I thnk it has more to do with the voltage the tubes operate at, rather than the frequencies they are amplifying. I don't think there is a problem until you start getting into much higher voltages than any audio amplifier uses, But at those voltages, tubes can emit Xrays. May be an issue with CRTs, not sure.
Lonestarsouth: This is utter nonsense!! Ionizing radiation only can occur when sub-atomic particles are excited by a high energy, high frequency power source. If your statement was true then electrostatic loudspeakers would have been ultra-deadly.
Actually, I believe that Honest1's statement about x-rays potentially being generated by high voltage tubes (much higher than any audio tube would use) is correct. As I understand it, an x-ray tube functions by bombarding the material in its plate with electrons that are emitted by a heated cathode, and then accelerated to high energy levels by means of a very high dc voltage applied to the plate. The voltage is typically in the range of 30,000 volts or more.
The analogy to electrostatic speakers is not applicable. While several thousand volts may be present between the elements of an esl, there is no electron emitter, no vacuum, and no plate being bombarded.
X-ray emission was definitely an issue with early tv sets, both from their crt's and from the tubes in their high voltage power supplies. Modern tv's and crt monitors use solid state power supplies, and presumably/supposedly limit x-ray emission from crt's via lead shielding which is impregnated into the glass.
See this wikipedia article for further information:
http://en.wikipedia.org/wiki/X-ray_tube
Regards,
-- Al