Hello St. Louisian,
One thing you could try to eliminate the shunt regulator stage as a problem is lifting the cap off the top of the 6BK4. You will have to keep the plate cap away from anything as it will have HV on it. You could also connect your HV probe to the 6BK4 plate cap to lessen the possibility of arcing. If that doesn't make any difference try scoping the control grids of the HO tubes to make sure the sawtooth peak to peak voltage is what it should be. Let us know what happens.

Jeff (PUNK ROCK DUDE)