I know 7-seg VFD is NOT for audio amplifier but digit display, and I am rather "crazy", but eventually I overcome "crazy howling" issue. Without any trick, triode operation 7-seg VFD will cry and howl up to maximum its output. Unless your house is like " Little house on the Prairie", your neighbour will get angry for continuous noisy howling.

What's the trick? It is indeed conventional way to stabilize howling of direct-heated tube, putting magnet on the tube.

The picture above shows the actual way to reduce howling. It is quite "size and position sensitive" and bigger magnet will kill its amplification, and smaller one will not have any effect for it. Anyway I found the way!