I would be interested in what "repairs and mods" you ultimately decide upon and why. Because, I may do the same.
Though, I agree, I'm not sure I would want to do anything to diminish the detail resolution, and clarity of the Sony... if... the tube mod does that. That's the reason I prefer the Sony to the Marantz - so, I wouldn't want a mod to make it sound more like a Marantz (though the Marantz does sound good).