I enlarge to 8x10 or 11x14 from 35mm negatives. Using Tri-X with Xtol 1:1, there is more grain than I would like on an 11x14 print that I really coned down on from the negative.
Enlarging a 36mm wide negative to 14" is about 10x enlargement. With Tri-X you should be seeing some grain about then. If you want a little less graininess, use the XTOL straight. If that's not enough for you then you might want to try TMY-2. If you want to stick with Tri-X you might want to move to a developer with more solvent characteristics. But this will cost you some real film speed and a little sharpness too.
I have done 11x14 before where most of the negative was used and I found the grain acceptable using Tri X or Neopan 400 shot at 320 and developed in D-76 1:1
This is perhaps true. D-76 will cost you some film speed. Maybe 1/2 to 2/3 stop. If you didn't compensate for this, then your overall exposure was less and your developed density would therefore also be less (compared to XTOL 1:1 which will give you somewhat higher film speed and therefore correspondingly higher density for an identical exposure).
Graininess is directly related to density. It's the metallic silver that makes up the films' density that also makes up the films' graininess.
I'm just saying that one explanation for what you are seeing comparing D-76 and XTOL is the differences in the real film speed and the resulting image density. When I've compared film developed with D-76, HC-110, and XTOL and compensated for the real film speed and developed to the same contrast index (as well as I could without access to a lab) I've seen very little difference between them. The XTOL was perhaps just a bit sharper and perhaps just a bit less grainy. But at 10x enlargement it's not much of a difference.
I plan on shooting indoors without a flash and want a B&W print film (35mm)/developer combo that can be enlarged to 11x14 with not too much grain (very subjective, I know).
In that case you'll probably be more interested in reciprocity failure in your shadows. Another reason to think about TMY-2 maybe.
I've read of Diafine with Tri-X rated at 1250 is a good pair. Is that the "best" combo or is Neopan 1600 rated at box speed or less with X tol or Delta 3200 with whatever developer better for less grain?
Diafine will give you a boost in real film speed. But the cost is serious graininess. I doubt you'll like it at 10x, if you don't like Tri-X at 10x already.
You may perhaps want to read up on the various capabilities of film developers. I like Anchell and Troops'
The Film Developing Cookbook for this myself. Pretty readable and it gives a good and fairly clear explanation of what the various developers do and don't do. More knowledge you can use to find the right tools to do the job you want done.