630 chassis low HV?

[N2IXK]

Restoring a Crosley 9-408 (RCA 630 chassis clone), and have the set working pretty well except for a somewhat dim image with excessive blooming.

HV checks at 8.2 kV with no load (Should be 9 kV per service data), but gets pulled down to around 6 kV when brightness is set to normal viewing level. Have tried swapping 1B3 and 6BG6 tubes to no avail. Drive signal and cathode current waveform on 6BG6 looks good. Am testing with a 5AXP4 test CRT, but adding additional HV filter capacitance to simulate the 10BP4 doesn't increase HV as I thought it might.

Any suggestions where to go from here? Width and horiz linearity look good, so sweep circuitry seems to be working pretty well.