You might get lucky with changing tubes or cables, but it's unlikely and it's just a chance. The best thing is to shield the phono stage. There are a variety of ways of doing this. Most RF can be shielded by a wire cage that is tied to ground. There are other shielding materials such as "mu" metal that are also very effective. I had the same problem in one home and was successful with a piece of mu metal above and below the unit. The best shields enclose the unit entirely, but just like an antena that works best at a certain location, shielding can often be "incomplete" and still get the job done.
