Recta-guru folklore has it the D getter works better, but they must be parallel to the nearest Ley Line for the best sonic stability, whereas the round ones will cause a massive disturbance in the space-time continuum and will no doubt cause the sonic image to become far less stable, in fact round getter GZ32’s were not used in the first TARDIS due to this stability problem.
There have been instances where a metal base GZ34’s has been fitted to Quad II’s, sadly this caused such a massive disturbance in the space-time continuum the listener disappeared up his own worm hole to another dimension
Terry