Mine can tell the difference between brands and freshness of hay. He loves his veggies so much, he isn't picky on that.

That's pretty limiting, so I wonder if it's a certain green in the salad that's something of a treat mixed in that they love so much.
Bunnies can be very particular with which greens they like. If you need to be able to buy other brands, and I recommend fresh produce more than a pre-mixed bag brand, do a little taste testing. You can feed each kind of green from a bag sample first, which they're hungry, and see if any are gobbled up the most. If so, that's the green that is really the prize in the cracker jacks box. Say it's a piece of kale, or a sprig of cilantro. In that case, you can just get some fresh bunch of that as a staple in their diet.
Try feeding small amounts of fresh greens to see what they like. Once you identify a few greens they both will eat routinely, you can buy it fresh or bagged from then on. Don't bother with iceberg lettuce, though. It's not good for bunnies.

I do think you need more variety - I agree with Pam that it must be more the mix than the brand. You should try things like cilantro, parsley, kale, dandelion greens, assorted heads of lettuces (not in the bag like romaine, butter lettuce, endive, raddicchio). The problem I see with those bagged/container mixes is that the red lettuces perish much more quickly.

I agree that you may need more of a mix, but I do also think they may be able to taste the difference in brands. I'm pregnant, so that has made my taste buds super sensitive to things I've never tasted before. I can taste the difference in iceburg lettuce brands (only I eat it--not my rabbit!). One kind even made me sick. It tasted like chemicals. My hubby couldn't taste a difference. After this experience, I'm wondering if my rabbit is just super sensitive to taste like I am right now.

Mine like that Earthbound stuff. I try to buy them regular heads of lettuce and they refuse to touch it. Picky rabbits.

My bunny loves a certain brand of organic lettuce heads, but hates the non-organic pesticide-filled lettuce heads.
Perhaps there is a taste difference. The organic lettuce does seem to have a more pure taste, and is different in color and texture.

Heads up there is a recall on Tanimura-Antle romaine lettuce!

They seem to like every thing in the mix because they grab random pieces.

SuperCheesetastic: I never thought about the pesticides. I guess that could be why they don't like the other brands.

My husband tried some of the rabbit lettuce. He said it tasted "earthy". I always just thought it was because they were baby lettuces and they liked the sweeter taste. But maybe it is the earthy taste. They do get a variety with the mixes. I always thought that that was good enough considering they don't like regular lettuce, cilantro, parsley, carrots..

I'd also check out farmer's markets organically grown natural greens. I have a few near where I live, so I always buy fresh produce for my bunny. But there are lots of Green Markets springing up by local farmers, so you may find some organic growers who sell near you.