which chassis out there has the better picture quality in 15hz
Well, they will all be about the same.

For a 25" I have to recommend a K7000.
Just reliable, easy to repair, etc.
BUT ..... you have to have a yoke that is compatible regardless of what chassis you use.
And the neckboard pinout needs to match as well.
Your other option would be to go with an aftermarket version that they can match to what you already have. (tube/yoke)