I think your 400 ISO exposed Tri-X looks fine and the 1600 looks underexposed - as expected. Also, keep in mind that Tri-X was around long before anyone thought about using scanners to scan negatives. One reason I use Silverfast is because it has presets for a number of films to help get in the ballpark, so to speak. Moreover, if you do not properly expose and develop, then you cannot be assured that every frame will capture the maximum shadows and highlights. The times provided for development are just approximations. If you want the perfect negative, then you need to expose for the shadows and process for the highlights. Often difficult to do with roll film.