Symphony is a blend, it will not fade because it is 65% poly but it will not shrink either, so keep that in mind.
Kona is always good but could be expensive for backing since you need quite a bit of yardage.
Country Classics are thinner then kona and their weave is not as tight, but that being said, I think it would be perfect for backing. I use it for sashing as well. All 100% cottons will fade with washing, especially black.
I backed a quilt with Symphony broadcloth once and it turned out fine, but it does have a very different feel than all cotton fabric. I have not used Legacy Studio solids so I cannot really give any opinion on that fabric. I hope this helps.