Why does my tube amp hit distortion at a low volume?

Question:

Why does my tube amp hit distortion at a low volume?

I have this old tube amp it has two preamp tubes two power tubes, and one rectifier tube. Beign that it is an old amp could it be the speaker? When im playing with single coils I get mild overdrive , but when I use my humbucker it goes into sudden overdrive at a low volume. Should I change my speaker to a higher wattage?? Its an old 1X12 amp built prior to the 50s.

Best Answer - Chosen by Asker: Edit:
Informative answer removed.
What a waste of time to give you a solid answer bubblehead.