Thanks.
You mentioned E-I lam-stacks 2x2, 3x3, etc; not flipped at every layer, but not butted. MagMet has a chart in a paper:
http://www.magmet.com/pdf/TransformDesignConsiderat.pdf
See page 4, Fig 10.
For zero DC, off the left side of the chart, straight 1x1 stack gives the best permeability; but 2x2 gives 80% perm with maybe 80% of the labor. In many small power transformers, low perm is not a problem but low cost wins the contract, just pointing that out.
If you have DC.... take a Champ output transformer. Say 2,500 turns and 40mA. That's 100 Ampere-Turns. Slide to the right, 100AT gives best perm with a good fat butt.
In audio SE OTs, the turns and the amps go together. Oh, if you needed NO bass (super-tweeter driver), thus low perm is OK, you might run near 10 AT, and then a 3x3 stack is better than either 1x1 or a butt. But that's an unusual case.
I think the iron you have been looking at is push-pull, nominally zero DC, but they want a 10% allowance for DC unbalance. That will tend to hit near 10AT and a 3x3 stack.
The exact places the several options cross will depend on the lam and core dimensions. This table is computed for 0.014" EE2425, whatever that is. (Anything like a Champ OT?)