I avoid white chickens because I free range all of my birds, and white ones make such nice targets for the local predators, they get picked off first, if there's a fox or anything around. Even Buff Orpingtons get picked off more easily, that bright gold just stands out. I tend to go for darker colors, for better camouflage. Multi-colored feathers let a bird "blend" better.
Many people prefer white birds because they look cleaner when plucked, the dark feathers leave colored spots in the skin, in a gel-like substance from the base of the feathers. The same stuff is left behind in the skin of white birds, you just don't see it, because there's no pigment.
The reason you got birds that didn't resemble the parent from your X roo, is what I said before, they don't breed true. They aren't a true breed, but a hybrid of specially developed strains.
I've done some reading about various breeds, and what crosses were used in the past to produce good table birds. Cornish roo x Dorking hen was the preferred mix in England, for decades. So I thought that would be a good place to start. Delawares also sound like a good possibility, because they're fairly chunky birds too, at least that's what I hear. I don't have any yet, so I'll know for sure, later. 
These birds will NOT be as fast growing as the Cornish X's you had. That's not my goal. I want sustainability, not just super fast fat birds. If you want super fat tender birds, stick to the C.X's. 
If you're wanting to caponize C/ X's to see if you can get them even bigger and fatter, I don't think your goal is the same as what I'm trying to do. Those C X's already tend to die of getting too big, too fast, I think they may already be at their max to live to around 8 weeks to butcher. It's unusual to have one live much past 14 weeks, though it happens now and then. I had some C X hens make it to almost a year, and they may have been C roasters, rather than the faster growing C X. I bought them from a farmer, who'd gotten them from a hatchery, the first year I was keeping chickens. 
Many moons ago!
I don't caponize, don't want to. The idea of subjecting a live, wide awake, fully aware bird or animal to surgery with no anesthesia, just makes me shudder. Especially now, when there are meat breeds that pretty much turn out the way caponized roos are supposed to turn out. If that's the body type you want, you may as well just go with that, instead of spending the time and money, and a lot of effort, in breeding. There are several really good meat strains available, the C x's, C roasters, red bros, broilers in white, red, or black. They aren't bad birds, they just aren't what 
I personally want.
I wouldn't use a white Cornish roo at all, because I prefer dark, or at least multi color, birds. I'd be more inclined to use dark Cornish roo over a Brahma hen. Buff if I had one, but what I have right now is the light Brahma hens. But, I don't know if that cross will result in anything like I'm looking for. As I said, this is all experimentation to me, at this stage. In 2 or 3 years, I might be able to give better info.  
If your goal is an acceptable table bird that you can keep as a sustainable breed, so you don't have to buy hatchery chicks, you're probably not going to have huge, fat birds like the C X's. For me, that's ok, I'm willing to trade off the extra weight for sustainability.
It sounds like you may want to do a bit more research on this subject before you jump in. I suggest reading everything you can find on the subject, before you invest in breeding stock for experiments. For breeding stock, I'm hunting for breeders of show quality stock, rather than production run hatchery stock. The difference in SQ and PR birds is amazing. I don't need SQ, as I'm not showing, but most breeders sell off the ones that don't quite meet color standards or the conformation may be a bit off. They're still much higher quality than hatchery stock, and I want good birds to start with. So this is a long, slow process. It can take a long time to just acquire the birds to breed in the first place. And, it's not likely you'll achieve your goal on the first try, either. I expect I'll have a number of misses before I get a hit.
Good luck, I hope you are able to achieve what you want!