Counterpoint 5.1- Beyond My Comprehension

I just experienced good news a moment ago- my Counterpoint 5.1 preamp is again operating properly. I had experienced greatly diminished volume in my system. This happened suddenly- when I powered everything up an placed an LP on the table, it was barley audible. All other selections were lower also.

To make a long story of tube swapping, amp swapping and speaker swapping short, I replaced the CA4 in the power supply and all is well. It appeared to have a smoky band around the center of the tube.

Does anyone know why this caused a reduction in volume instead of a total loss of volume?

Thanks in advance!