Theaudiotweak..."Tube" preamp you say. Perhaps you should be concerned about the effect of a magnet on the electron beams in your tubes.
At work, when testing a vidicon to calibrate it we found that the results varied depending on which way the device was pointed. (A vidicon is a vacuum tube imaging device, once used for TV). Turns out that the earth's magnetic field was deflecting the electron beam. Of course we all know what a speaker magnet does to a CRT. However, the good news is that, unlike transistors, tubes are not damaged by radiation.