EDIT: WHOOPS, forgot that scanning is off limits, so go to
http://DPUG.org for more scanning questions if you need further info, sorry about that folks, I forgot!
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Look into the company "betterscanning" they make special film holders for scanners with ANR Glass placed on top of the film to keep it flat and much more adjustable holders then most that come with standard scanning equipment.
Different film bases have different thicknesses but more importantly react to temperature differently, one film might bow or arch differently then another film meaning that the film plane is at a different level, so for example your FP4+ Might lineup perfectly on the film plane with the scanner lens, whereas the Tri-X might be forward or backward more because of film bowing, and so the distance between the lens and the film is different and it is out of focus just like using a camera.
You also have to consider that color film at 400 speed will probably still be sharper then Tri-X for various reasons.
Finally, although DD-X is an excellent developer which I use myself, in particular it does really well for shadow detail and bringing highlights down as to not blow them out, even if user error occurs in the exposure, however it is not a fine grain type developer, it is meant to be partnered with Tmax400 or Delta films, which are inherently more fine grained then Tri-X which mitigates any sharpness issues using this developer. This developer is also particularly good with HP5+ Which is similar in grain style to Tri-X, but when paired with either, it's benefit is not it's fine grained nature but rather its ability to push films very well, for example HP5+ shot and pushed for 3200 looks like this...
Keep in mind, this countertop is completely black, and of course this is inside, see how the shadows even in the darkest areas are not totally into the black and there aren't really any blonde highlights even on the completely white cruet.
HP5+ @ 3200 | DD-X
So I suspect you might be dealing with a scanner issue more than anything, of course even if the film is slightly less sharp, if your scanner is off, that will only accentuate any already obvious differences between a tighter grained film like FP4+ vs Tri-X and make it even more un-sharp.
PS, only change one factor at a time, don't try and change your developer while also changing your scanner film to lass height, etc, or it will be hard to identify the true culprit.
Good luck!
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EDIT: WHOOPS, forgot that scanning is off limits, so go to
http://DPUG.org for more scanning questions if you need further info, sorry about that folks, I forgot!