So here's the update in quick bullet points.
What's working:
- Horizontal sync - strong, stays locked in at center of control rotation
- Audio - full and loud (when using BT Agile modulator)
- High Voltage (measures 17.5kV)
- CRT has responsive cutoff and good emissions as tested by CR70. Output is bright
- Odd video/audio "noise" has stopped (unclear why)
What's not working:
- AGC control non-responsive
- Contrast very poor
- Vertical hold poor -- worse as set warms up
- Picture has a "wobble" -- especially visible with test pattern (https://youtu.be/XO5ciFJxHiM)
The Sams troubleshooting notes keep pointing at V7-6AN8 (Sync Amp), V8-6AN8 (AGC keying/Vert Mult) and V6-12BY7 (video output). Have tried four different 6AN8 tubes, and two 12BY7's -- however I've just ordered some more of both.
At this point, is there anything else this could be other than those tubes or anything else I could try?
Thanks again everyone, for all of your help and great suggestions!
FK