It's hard to trust any app when it comes to ... down-bit'ing (not sure how to name that).
It's not clear to me that those apps have really understood the Fourier theorem, or how, according to that theorem you can somewhat prepare such conversion beforehand. (I'm saying this but yeah, trying to read about this on wikipedia made me fell asleep way before i understood anything).
I've heard really good sounding piano turn into a thundertorm of clippings just going from 24bit to 16. Some conversions do just work easily though.. most even.
...
Anyways, having said that... I don't know that many now widely known as "wave editors" as much as I would wish, but Peak should be able to do that (efficiently or not
)
There's a really stripped down version here, but I think you coud use it for that..