Sorry, there's no secret. I use Duncan Hines Butter Recipe cake mix. The only thing I add is a little almond flavoring -- just a hint of it.
Sometimes there is a request for the WHITE cake mix. IMHO, it has "no taste" -- kinda tastes like the box. :lol: So I use the whole egg and tell them that it will not be "real white." Then I do the same thing with the hint of almond flavoring.
As far as the denseness goes, it's a tender fine texture, so I use a lot of support (dowel rods, etc) to keep the cake from collapsing. Knock on wood, I've always been very blessed. There is a recipe that can be used You may have heard of the Dream Cake. It's a recipe using the powdered Dream Whip. It gives a firmer cake -- a little like a pound cake.