I would think it's possible to influence the plate current with a magnetic field - after all you can use a magnetic field as an accelerator.Magnitude and direction are the issues of course, but it might be fun to play with.
Probably a good indicator would be how close you can get the top of the speaker to a computer monitor before you see distortion - if you can touch the monitor with the top of the speaker without a problem then it would seem reasonable to assume you wouldn't be deflecting electrons in the amp's tubes. Not that would harm anything in any event.