Although not related to the quirky measurement of a 15V lower B+ before the choke filter, what advice did you follow with respect to adding the ss diodes. Imho those diodes should preferably be 1N4007 (not UF4007 as they have a lower PIV), and you should use 2x in series for each valve anode (not 1x) due to a relatively high 360V secondary winding voltage. In addition it is better to not mount the ss diodes to spare pins of the rectifier valve socket due to the high voltage between pins, and to ensure you have cleaned between the socket pins to remove any dust that may cause arcing/tracking.
One influence for the quirky voltage reading could be a change in mains voltage - but you would need to make before/after measurements of say heater voltage to confirm/exclude that influence (eg. by doing a temporary bridge across the ss diode).
That's what I was thinking. Is a 3.5% change. My wall voltage changes that much (say 125V to 120V) all day long. A single 12A load will drop my line 4.8V. More on laundry day (13A well-pump, 21A heater, 17A dryer). In such a case you want a dedicated wall monitor.
Very different from my old shop. After the backhoe blessed one of 2 power feeds to the city, I had a VERY steady 109V day and night. I had to correct all my power readings (or lug the Variac from another building where it was needed more).