I can sympathize I have 2 VERY picky eaters too! We've been thru a slew of different foods....they turn their nose up at the premium brands I offer...but go to my sisters and gobble up her lower quality food
...Like you said earlier..it's probably like Mcdonalds to them! But not necessarily nutritious.
A store here in town offers free samples of their premium foods...you might check around and see if you have a shop close by that offers free samples...you might find a brand they like. Don't know if they have feed and grain stores where you live..but they also tend to carry premium foods and offer samples.
We just switched to The great life and they seem to really like it! Keeping my fingers crossed.. also feed them a little of Evo.
Good luck!