You'll probably never get the 15kHz mode quite perfect... geometry and color saturation/purity are the 2 biggest issues with those chassis as you probably well know. It's been years since I had my Impress that I put a C3129A on and I cant recall if it had additional "master" pots on the chassis itself... but if it does, try adjusting those first under load and doing the micro adjustments on the remote board.
Guess who is about to get some mf-ing vertical shit up in here??