Very interesting thread! I do not own any tube amps myself, but I have asked and been advised over time by 2 separate tube-amp manufacturers on this exact subject.
In both cases I was advised, that indeed some x-ray radiation is produced by the higher voltage tube-amps (but not from low-voltage tube linestages e.g. preamps and dacs etc). They do NOT prefer to talk about this aspect! Most (including the end-users) will, for understandable reasons, accordingly try to laugh it off / dismiss it as a paranoid non-issue. I was told the concern is roughly equivalent to the exposure from the older CRT-style TV sets - which also can produce x-ray radiation as they age and gradually get out of proper specification. The absolute energy level from tube-amps is not likely very high (especially from over 20 feet away), but long-term user (and/or family) exposure to even low-level x-ray radiation is in fact, now showing to be a likely health risk - especially as the damage builds in the body over time, and adds to other more common radiation exposures received throughout our lives.
Also, some of the older NOS tubes (especially the Russian variety) actually also contain radioactive material which improves the ionization process, and thus have radiation warnings & symbols on their packaging. If such a tube breaks in one's home, I was advised this can be a genuine risk as well.
I am not personally an expert on the subject, but only repeating the info that I've personally received - as I was extremely surprised to find this particular topic discussed here.
