FWIW my response will at least give you a "bump", if nothing else...
Your Q is a difficult one for a few reasons. Not the least of which is that tone is subjective. One mans "fat" is another mans "flabby". "Bright is "harsh" etc.
Primary impedance probably effects the feel of an amp more than the tone. Which is also subjective. Marshall did very well with EL34 amps that ran under 4k primary. And the TW Express runs a pair at 6.6k.
Plus, there are many other aspects of transformer design that effect tone and feel. The core stack size/material, the spacings, interleaving, wire guage, etc. By the time it's done the alchemy can bruise my wittle cranium pretty badly. There are ALOT of opinions out there and the good solid facts seem hard to come by. Probably because so many things can change as you mix up the formula. So you can't really say "A bigger core stack equals better low end." because the other design elements can interfere.
If your consideration is to order a custom OT to suit your desires, It's MHO that it's a bad idea. Most of the transformers being designed for guitar amps are clones or slightly modified versions. I think the reasons are many. For one thing, anyone designing OTs from scratch is trying to make the flatest cleanest response unit he/she can for the materials/budget that they have to work with. Most guitar amp OTs are near replicas of previous designs that are considered good sounding. In the day when tubes were "it", guitar amp OTs were "off the shelf" units from average mfgs. There level of quality was usually based on budget. It is the inherant flaws in these units that some consider to be the "mojo" of their tone. Designing transformers with intentional failings has never been the goal at any time in history before now. Now we want to recreate the old mojo using new materials. But it's still alchemy at this point. We can reverse engineer a good guitar amp OT. But it's alot tougher trying to design one from scratch.
Considering most amps built in the hayday of tube guitar amps were designed with off the shelf OTs and then tweaked in. And considering that thats what most of us here do. I have to think that it's currently the best approach. Choose an amp design that you like the sound of and buy a suitable OT for that amp. Then tweak the circuit to your hearts content. If after that you think one of the OT parameters could be changed to idealize, then you could try it. But trying to come up with a custom design OT before you have an amp is like putting the horse behind the cart.